SQL is a standard computer language for accessing and ...
SQL Tutorial
|[pic] |SQL is a standard computer language for accessing and manipulating databases. |
| |In this tutorial you will learn how to use SQL to access and manipulate data in Oracle, |
| |Sybase, SQL Server, DB2, Access, and other database systems. |
Introduction to SQL
SQL is a standard computer language for accessing and manipulating databases.
What is SQL?
• SQL stands for Structured Query Language
• SQL allows you to access a database
• SQL is an ANSI standard computer language
• SQL can execute queries against a database
• SQL can retrieve data from a database
• SQL can insert new records in a database
• SQL can delete records from a database
• SQL can update records in a database
• SQL is easy to learn
SQL is a Standard - BUT....
SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc.
Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others).
Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard!
SQL Database Tables
A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data.
Below is an example of a table called "Persons":
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City).
SQL Queries
With SQL, we can query a database and have a result set returned.
A query like this:
|SELECT LastName FROM Persons |
Gives a result set like this:
|LastName |
|Hansen |
|Svendson |
|Pettersen |
Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials.
SQL Data Manipulation Language (DML)
SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records.
These query and update commands together form the Data Manipulation Language (DML) part of SQL:
▪ SELECT - extracts data from a database table
▪ UPDATE - updates data in a database table
▪ DELETE - deletes data from a database table
▪ INSERT INTO - inserts new data into a database table
SQL Data Definition Language (DDL)
The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables.
The most important DDL statements in SQL are:
▪ CREATE TABLE - creates a new database table
▪ ALTER TABLE - alters (changes) a database table
▪ DROP TABLE - deletes a database table
▪ CREATE INDEX - creates an index (search key)
▪ DROP INDEX - deletes an index
SQL The SELECT Statement
The SELECT Statement
The SELECT statement is used to select data from a table. The tabular result is stored in a result table (called the result-set).
Syntax
|SELECT column_name(s) |
|FROM table_name |
Select Some Columns
To select the columns named "LastName" and "FirstName", use a SELECT statement like this:
|SELECT LastName,FirstName FROM Persons |
"Persons" table
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
Result
|LastName |FirstName |
|Hansen |Ola |
|Svendson |Tove |
|Pettersen |Kari |
Select All Columns
To select all columns from the "Persons" table, use a * symbol instead of column names, like this:
|SELECT * FROM Persons |
Result
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
The Result Set
The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-First-Record, Get-Record-Content, Move-To-Next-Record, etc.
Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial.
Semicolon after SQL Statements?
Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
Some SQL tutorials end each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.
The SELECT DISTINCT Statement
The DISTINCT keyword is used to return only distinct (different) values.
The SELECT statement returns information from table columns. But what if we only want to select distinct elements?
With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement:
Syntax
|SELECT DISTINCT column_name(s) |
|FROM table_name |
Using the DISTINCT keyword
To select ALL values from the column named "Company" we use a SELECT statement like this:
|SELECT Company FROM Orders |
"Orders" table
|Company |OrderNumber |
|Sega |3412 |
|W3Schools |2312 |
|Trio |4678 |
|W3Schools |6798 |
Result
|Company |
|Sega |
|W3Schools |
|Trio |
|W3Schools |
Note that "W3Schools" is listed twice in the result-set.
To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this:
|SELECT DISTINCT Company FROM Orders |
Result:
|Company |
|Sega |
|W3Schools |
|Trio |
Now "W3Schools" is listed only once in the result-set.
SQL The WHERE Clause
The WHERE clause is used to specify a selection criterion.
The WHERE Clause
To conditionally select data from a table, a WHERE clause can be added to the SELECT statement.
Syntax
|SELECT column FROM table |
|WHERE column operator value |
With the WHERE clause, the following operators can be used:
|Operator |Description |
|= |Equal |
| |Not equal |
|> |Greater than |
|< |Less than |
|>= |Greater than or equal |
|1965 |
|This is wrong: |
|SELECT * FROM Persons WHERE Year>'1965' |
The LIKE Condition
The LIKE condition is used to specify a search for a pattern in a column.
Syntax
|SELECT column FROM table |
|WHERE column LIKE pattern |
A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
Using LIKE
The following SQL statement will return persons with first names that start with an 'O':
|SELECT * FROM Persons |
|WHERE FirstName LIKE 'O%' |
The following SQL statement will return persons with first names that end with an 'a':
|SELECT * FROM Persons |
|WHERE FirstName LIKE '%a' |
The following SQL statement will return persons with first names that contain the pattern 'la':
|SELECT * FROM Persons |
|WHERE FirstName LIKE '%la%' |
SQL The INSERT INTO Statement
The INSERT INTO Statement
The INSERT INTO statement is used to insert new rows into a table.
Syntax
|INSERT INTO table_name |
|VALUES (value1, value2,....) |
You can also specify the columns for which you want to insert data:
|INSERT INTO table_name (column1, column2,...) |
|VALUES (value1, value2,....) |
Insert a New Row
This "Persons" table:
|LastName |FirstName |Address |City |
|Pettersen |Kari |Storgt 20 |Stavanger |
And this SQL statement:
|INSERT INTO Persons |
|VALUES ('Hetland', 'Camilla', 'Hagabakka 24', 'Sandnes') |
Will give this result:
|LastName |FirstName |Address |City |
|Pettersen |Kari |Storgt 20 |Stavanger |
|Hetland |Camilla |Hagabakka 24 |Sandnes |
Insert Data in Specified Columns
This "Persons" table:
|LastName |FirstName |Address |City |
|Pettersen |Kari |Storgt 20 |Stavanger |
|Hetland |Camilla |Hagabakka 24 |Sandnes |
And This SQL statement:
|INSERT INTO Persons (LastName, Address) |
|VALUES ('Rasmussen', 'Storgt 67') |
Will give this result:
|LastName |FirstName |Address |City |
|Pettersen |Kari |Storgt 20 |Stavanger |
|Hetland |Camilla |Hagabakka 24 |Sandnes |
|Rasmussen | |Storgt 67 | |
SQL The UPDATE Statement
The Update Statement
The UPDATE statement is used to modify the data in a table.
Syntax
|UPDATE table_name |
|SET column_name = new_value |
|WHERE column_name = some_value |
Person:
|LastName |FirstName |Address |City |
|Nilsen |Fred |Kirkegt 56 |Stavanger |
|Rasmussen | |Storgt 67 | |
Update one Column in a Row
We want to add a first name to the person with a last name of "Rasmussen":
|UPDATE Person SET FirstName = 'Nina' |
|WHERE LastName = 'Rasmussen' |
Result:
|LastName |FirstName |Address |City |
|Nilsen |Fred |Kirkegt 56 |Stavanger |
|Rasmussen |Nina |Storgt 67 | |
Update several Columns in a Row
We want to change the address and add the name of the city:
|UPDATE Person |
|SET Address = 'Stien 12', City = 'Stavanger' |
|WHERE LastName = 'Rasmussen' |
Result:
|LastName |FirstName |Address |City |
|Nilsen |Fred |Kirkegt 56 |Stavanger |
|Rasmussen |Nina |Stien 12 |Stavanger |
SQL The Delete Statement
The Delete Statement
The DELETE statement is used to delete rows in a table.
Syntax
|DELETE FROM table_name |
|WHERE column_name = some_value |
Person:
|LastName |FirstName |Address |City |
|Nilsen |Fred |Kirkegt 56 |Stavanger |
|Rasmussen |Nina |Stien 12 |Stavanger |
Delete a Row
"Nina Rasmussen" is going to be deleted:
|DELETE FROM Person WHERE LastName = 'Rasmussen' |
Result
|LastName |FirstName |Address |City |
|Nilsen |Fred |Kirkegt 56 |Stavanger |
Delete All Rows
It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
|DELETE FROM table_name |
|or |
|DELETE * FROM table_name |
SQL Try It
Test your SQL Skills
On this page you can test your SQL skills.
We will use the Customers table in the Northwind database:
|CompanyName |ContactName |Address |City |
|Alfreds Futterkiste |Maria Anders |Obere Str. 57 |Berlin |
|Berglunds snabbköp |Christina Berglund |Berguvsvägen 8 |Luleå |
|Centro comercial Moctezuma |Francisco Chang |Sierras de Granada 9993 |México D.F. |
|Ernst Handel |Roland Mendel |Kirchgasse 6 |Graz |
|FISSA Fabrica Inter. Salchichas S.A. |Diego Roel |C/ Moralzarzal, 86 |Madrid |
|Galería del gastrónomo |Eduardo Saavedra |Rambla de Cataluña, 23 |Barcelona |
|Island Trading |Helen Bennett |Garden House Crowther Way |Cowes |
|Königlich Essen |Philip Cramer |Maubelstr. 90 |Brandenburg |
|Laughing Bacchus Wine Cellars |Yoshi Tannamuri |1900 Oak St. |Vancouver |
|Magazzini Alimentari Riuniti |Giovanni Rovelli |Via Ludovico il Moro 22 |Bergamo |
|North/South |Simon Crowther |South House 300 Queensbridge |London |
|Paris spécialités |Marie Bertrand |265, boulevard Charonne |Paris |
|Rattlesnake Canyon Grocery |Paula Wilson |2817 Milton Dr. |Albuquerque |
|Simons bistro |Jytte Petersen |Vinbæltet 34 |København |
|The Big Cheese |Liz Nixon |89 Jefferson Way Suite 2 |Portland |
|Vaffeljernet |Palle Ibsen |Smagsløget 45 |Århus |
|Wolski Zajazd |Zbyszek Piestrzeniewicz |ul. Filtrowa 68 |Warszawa |
To preserve space, the table above is a subset of the Customers table used in the example below.
Try it Yourself
To see how SQL works, you can copy the SQL statements below and paste them into the textarea, or you can make your own SQL statements.
|SELECT * FROM customers |
|SELECT CompanyName, ContactName |
|FROM customers |
|SELECT * FROM customers |
|WHERE companyname LIKE 'a%' |
|SELECT CompanyName, ContactName |
|FROM customers |
|WHERE CompanyName > 'g' |
|AND ContactName > 'g' |
[pic]
SQL ORDER BY
The ORDER BY keyword is used to sort the result.
Sort the Rows
The ORDER BY clause is used to sort the rows.
Orders:
|Company |OrderNumber |
|Sega |3412 |
|ABC Shop |5678 |
|W3Schools |2312 |
|W3Schools |6798 |
Example
To display the companies in alphabetical order:
|SELECT Company, OrderNumber FROM Orders |
|ORDER BY Company |
Result:
|Company |OrderNumber |
|ABC Shop |5678 |
|Sega |3412 |
|W3Schools |6798 |
|W3Schools |2312 |
Example
To display the companies in alphabetical order AND the ordernumbers in numerical order:
|SELECT Company, OrderNumber FROM Orders |
|ORDER BY Company, OrderNumber |
Result:
|Company |OrderNumber |
|ABC Shop |5678 |
|Sega |3412 |
|W3Schools |2312 |
|W3Schools |6798 |
Example
To display the companies in reverse alphabetical order:
|SELECT Company, OrderNumber FROM Orders |
|ORDER BY Company DESC |
Result:
|Company |OrderNumber |
|W3Schools |6798 |
|W3Schools |2312 |
|Sega |3412 |
|ABC Shop |5678 |
Example
To display the companies in reverse alphabetical order AND the ordernumbers in numerical order:
|SELECT Company, OrderNumber FROM Orders |
|ORDER BY Company DESC, OrderNumber ASC |
Result:
|Company |OrderNumber |
|W3Schools |2312 |
|W3Schools |6798 |
|Sega |3412 |
|ABC Shop |5678 |
SQL AND & OR
AND & OR
AND and OR join two or more conditions in a WHERE clause.
The AND operator displays a row if ALL conditions listed are true. The OR operator displays a row if ANY of the conditions listed are true.
Original Table (used in the examples)
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Svendson |Stephen |Kaivn 18 |Sandnes |
Example
Use AND to display each person with the first name equal to "Tove", and the last name equal to "Svendson":
|SELECT * FROM Persons |
|WHERE FirstName='Tove' |
|AND LastName='Svendson' |
Result:
|LastName |FirstName |Address |City |
|Svendson |Tove |Borgvn 23 |Sandnes |
Example
Use OR to display each person with the first name equal to "Tove", or the last name equal to "Svendson":
|SELECT * FROM Persons |
|WHERE firstname='Tove' |
|OR lastname='Svendson' |
Result:
|LastName |FirstName |Address |City |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Svendson |Stephen |Kaivn 18 |Sandnes |
Example
You can also combine AND and OR (use parentheses to form complex expressions):
|SELECT * FROM Persons WHERE |
|(FirstName='Tove' OR FirstName='Stephen') |
|AND LastName='Svendson' |
Result:
|LastName |FirstName |Address |City |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Svendson |Stephen |Kaivn 18 |Sandnes |
SQL IN
IN
The IN operator may be used if you know the exact value you want to return for at least one of the columns.
|SELECT column_name FROM table_name |
|WHERE column_name IN (value1,value2,..) |
Original Table (used in the examples)
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Nordmann |Anna |Neset 18 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
|Svendson |Tove |Borgvn 23 |Sandnes |
Example 1
To display the persons with LastName equal to "Hansen" or "Pettersen", use the following SQL:
|SELECT * FROM Persons |
|WHERE LastName IN ('Hansen','Pettersen') |
Result:
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
SQL BETWEEN
BETWEEN ... AND
The BETWEEN ... AND operator selects a range of data between two values. These values can be numbers, text, or dates.
|SELECT column_name FROM table_name |
|WHERE column_name |
|BETWEEN value1 AND value2 |
Original Table (used in the examples)
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Nordmann |Anna |Neset 18 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
|Svendson |Tove |Borgvn 23 |Sandnes |
Example 1
To display the persons alphabetically between (and including) "Hansen" and exclusive "Pettersen", use the following SQL:
|SELECT * FROM Persons WHERE LastName |
|BETWEEN 'Hansen' AND 'Pettersen' |
Result:
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Nordmann |Anna |Neset 18 |Sandnes |
IMPORTANT! The BETWEEN...AND operator is treated differently in different databases. With some databases a person with the LastName of "Hansen" or "Pettersen" will not be listed (BETWEEN..AND only selects fields that are between and excluding the test values). With some databases a person with the last name of "Hansen" or "Pettersen" will be listed (BETWEEN..AND selects fields that are between and including the test values). With other databases a person with the last name of "Hansen" will be listed, but "Pettersen" will not be listed (BETWEEN..AND selects fields between the test values, including the first test value and excluding the last test value). Therefore: Check how your database treats the BETWEEN....AND operator!
Example 2
To display the persons outside the range used in the previous example, use the NOT operator:
|SELECT * FROM Persons WHERE LastName |
|NOT BETWEEN 'Hansen' AND 'Pettersen' |
Result:
|LastName |FirstName |Address |City |
|Pettersen |Kari |Storgt 20 |Stavanger |
|Svendson |Tove |Borgvn 23 |Sandnes |
SQL Aliases
With SQL, aliases can be used for column names and table names.
Column Name Alias
The syntax is:
|SELECT column AS column_alias FROM table |
Table Name Alias
The syntax is:
|SELECT column FROM table AS table_alias |
Example: Using a Column Alias
This table (Persons):
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
And this SQL:
|SELECT LastName AS Family, FirstName AS Name |
|FROM Persons |
Returns this result:
|Family |Name |
|Hansen |Ola |
|Svendson |Tove |
|Pettersen |Kari |
Example: Using a Table Alias
This table (Persons):
|LastName |FirstName |Address |City |
|Hansen |Ola |Timoteivn 10 |Sandnes |
|Svendson |Tove |Borgvn 23 |Sandnes |
|Pettersen |Kari |Storgt 20 |Stavanger |
And this SQL:
|SELECT LastName, FirstName |
|FROM Persons AS Employees |
Returns this result:
Table Employees:
|LastName |FirstName |
|Hansen |Ola |
|Svendson |Tove |
|Pettersen |Kari |
SQL Join
Joins and Keys
Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
Tables in a database can be related to each other with keys. A primary key is a column with a unique value for each row. The purpose is to bind data together, across tables, without repeating all of the data in every table.
In the "Employees" table below, the "Employee_ID" column is the primary key, meaning that no two rows can have the same Employee_ID. The Employee_ID distinguishes two persons even if they have the same name.
When you look at the example tables below, notice that:
• The "Employee_ID" column is the primary key of the "Employees" table
• The "Prod_ID" column is the primary key of the "Orders" table
• The "Employee_ID" column in the "Orders" table is used to refer to the persons in the "Employees" table without using their names
Employees:
|Employee_ID |Name |
|01 |Hansen, Ola |
|02 |Svendson, Tove |
|03 |Svendson, Stephen |
|04 |Pettersen, Kari |
Orders:
|Prod_ID |Product |Employee_ID |
|234 |Printer |01 |
|657 |Table |03 |
|865 |Chair |03 |
Referring to Two Tables
We can select data from two tables by referring to two tables, like this:
Example
Who has ordered a product, and what did they order?
|SELECT Employees.Name, Orders.Product |
|FROM Employees, Orders |
|WHERE Employees.Employee_ID=Orders.Employee_ID |
Result
|Name |Product |
|Hansen, Ola |Printer |
|Svendson, Stephen |Table |
|Svendson, Stephen |Chair |
Example
Who ordered a printer?
|SELECT Employees.Name |
|FROM Employees, Orders |
|WHERE Employees.Employee_ID=Orders.Employee_ID |
|AND Orders.Product='Printer' |
Result
|Name |
|Hansen, Ola |
Using Joins
OR we can select data from two tables with the JOIN keyword, like this:
Example INNER JOIN
Syntax
|SELECT field1, field2, field3 |
|FROM first_table |
|INNER JOIN second_table |
|ON first_table.keyfield = second_table.foreign_keyfield |
Who has ordered a product, and what did they order?
|SELECT Employees.Name, Orders.Product |
|FROM Employees |
|INNER JOIN Orders |
|ON Employees.Employee_ID=Orders.Employee_ID |
The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Employees that do not have matches in Orders, those rows will not be listed.
Result
|Name |Product |
|Hansen, Ola |Printer |
|Svendson, Stephen |Table |
|Svendson, Stephen |Chair |
Example LEFT JOIN
Syntax
|SELECT field1, field2, field3 |
|FROM first_table |
|LEFT JOIN second_table |
|ON first_table.keyfield = second_table.foreign_keyfield |
List all employees, and their orders - if any.
|SELECT Employees.Name, Orders.Product |
|FROM Employees |
|LEFT JOIN Orders |
|ON Employees.Employee_ID=Orders.Employee_ID |
The LEFT JOIN returns all the rows from the first table (Employees), even if there are no matches in the second table (Orders). If there are rows in Employees that do not have matches in Orders, those rows also will be listed.
Result
|Name |Product |
|Hansen, Ola |Printer |
|Svendson, Tove | |
|Svendson, Stephen |Table |
|Svendson, Stephen |Chair |
|Pettersen, Kari | |
Example RIGHT JOIN
Syntax
|SELECT field1, field2, field3 |
|FROM first_table |
|RIGHT JOIN second_table |
|ON first_table.keyfield = second_table.foreign_keyfield |
List all orders, and who has ordered - if any.
|SELECT Employees.Name, Orders.Product |
|FROM Employees |
|RIGHT JOIN Orders |
|ON Employees.Employee_ID=Orders.Employee_ID |
The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Employees). If there had been any rows in Orders that did not have matches in Employees, those rows also would have been listed.
Result
|Name |Product |
|Hansen, Ola |Printer |
|Svendson, Stephen |Table |
|Svendson, Stephen |Chair |
Example
Who ordered a printer?
|SELECT Employees.Name |
|FROM Employees |
|INNER JOIN Orders |
|ON Employees.Employee_ID=Orders.Employee_ID |
|WHERE Orders.Product = 'Printer' |
Result
|Name |
|Hansen, Ola |
SQL UNION and UNION ALL
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type.
Note: With UNION, only distinct values are selected.
|SQL Statement 1 |
|UNION |
|SQL Statement 2 |
Employees_Norway:
|Employee_ID |E_Name |
|01 |Hansen, Ola |
|02 |Svendson, Tove |
|03 |Svendson, Stephen |
|04 |Pettersen, Kari |
Employees_USA:
|Employee_ID |E_Name |
|01 |Turner, Sally |
|02 |Kent, Clark |
|03 |Svendson, Stephen |
|04 |Scott, Stephen |
Using the UNION Command
Example
List all different employee names in Norway and USA:
|SELECT E_Name FROM Employees_Norway |
|UNION |
|SELECT E_Name FROM Employees_USA |
Result
|Name |
|Hansen, Ola |
|Svendson, Tove |
|Svendson, Stephen |
|Pettersen, Kari |
|Turner, Sally |
|Kent, Clark |
|Scott, Stephen |
Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them is listed. The UNION command only selects distinct values.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
|SQL Statement 1 |
|UNION ALL |
|SQL Statement 2 |
Using the UNION ALL Command
Example
List all employees in Norway and USA:
|SELECT E_Name FROM Employees_Norway |
|UNION ALL |
|SELECT E_Name FROM Employees_USA |
Result
|Name |
|Hansen, Ola |
|Svendson, Tove |
|Svendson, Stephen |
|Pettersen, Kari |
|Turner, Sally |
|Kent, Clark |
|Svendson, Stephen |
|Scott, Stephen |
SQL Create Database, Table, and Index
Create a Database
To create a database:
|CREATE DATABASE database_name |
Create a Table
To create a table in a database:
|CREATE TABLE table_name |
|( |
|column_name1 data_type, |
|column_name2 data_type, |
|....... |
|) |
Example
This example demonstrates how you can create a table named "Person", with four columns. The column names will be "LastName", "FirstName", "Address", and "Age":
|CREATE TABLE Person |
|( |
|LastName varchar, |
|FirstName varchar, |
|Address varchar, |
|Age int |
|) |
This example demonstrates how you can specify a maximum length for some columns:
|CREATE TABLE Person |
|( |
|LastName varchar(30), |
|FirstName varchar, |
|Address varchar, |
|Age int(3) |
|) |
The data type specifies what type of data the column can hold. The table below contains the most common data types in SQL:
|Data Type |Description |
|integer(size) |Hold integers only. The maximum number of digits are specified in parenthesis. |
|int(size) | |
|smallint(size) | |
|tinyint(size) | |
|decimal(size,d) |Hold numbers with fractions. The maximum number of digits are specified in "size". The |
|numeric(size,d) |maximum number of digits to the right of the decimal is specified in "d". |
|char(size) |Holds a fixed length string (can contain letters, numbers, and special characters). The |
| |fixed size is specified in parenthesis. |
|varchar(size) |Holds a variable length string (can contain letters, numbers, and special characters). The|
| |maximum size is specified in parenthesis. |
|date(yyyymmdd) |Holds a date |
Create Index
Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes, they are just used to speed up queries.
Note: Updating a table containing indexes takes more time than updating a table without, this is because the indexes also need an update. So, it is a good idea to create indexes only on columns that are often used for a search.
A Unique Index
Creates a unique index on a table. A unique index means that two rows cannot have the same index value.
|CREATE UNIQUE INDEX index_name |
|ON table_name (column_name) |
The "column_name" specifies the column you want indexed.
A Simple Index
Creates a simple index on a table. When the UNIQUE keyword is omitted, duplicate values are allowed.
|CREATE INDEX index_name |
|ON table_name (column_name) |
The "column_name" specifies the column you want indexed.
Example
This example creates a simple index, named "PersonIndex", on the LastName field of the Person table:
|CREATE INDEX PersonIndex |
|ON Person (LastName) |
If you want to index the values in a column in descending order, you can add the reserved word DESC after the column name:
|CREATE INDEX PersonIndex |
|ON Person (LastName DESC) |
If you want to index more than one column you can list the column names within the parentheses, separated by commas:
|CREATE INDEX PersonIndex |
|ON Person (LastName, FirstName) |
SQL Drop Index, Table and Database
Drop Index
You can delete an existing index in a table with the DROP statement.
|DROP INDEX table_name.index_name |
Delete a Table or Database
To delete a table (the table structure, attributes, and indexes will also be deleted):
|DROP TABLE table_name |
To delete a database:
|DROP DATABASE database_name |
Truncate a Table
What if we only want to get rid of the data inside a table, and not the table itself? Use the TRUNCATE TABLE command (deletes only the data inside the table):
|TRUNCATE TABLE table_name |
SQL ALTER TABLE
ALTER TABLE
The ALTER TABLE statement is used to add or drop columns in an existing table.
|ALTER TABLE table_name |
|ADD column_name datatype |
|ALTER TABLE table_name |
|DROP COLUMN column_name |
Note: Some database systems don't allow the dropping of a column in a database table (DROP COLUMN column_name).
Person:
|LastName |FirstName |Address |
|Pettersen |Kari |Storgt 20 |
Example
To add a column named "City" in the "Person" table:
|ALTER TABLE Person ADD City varchar(30) |
Result:
|LastName |FirstName |Address |City |
|Pettersen |Kari |Storgt 20 | |
Example
To drop the "Address" column in the "Person" table:
|ALTER TABLE Person DROP COLUMN Address |
Result:
|LastName |FirstName |City |
|Pettersen |Kari | |
SQL Functions
SQL has a lot of built-in functions for counting and calculations.
Function Syntax
The syntax for built-in SQL functions is:
|SELECT function(column) FROM table |
Types of Functions
There are several basic types and categories of functions in SQL. The basic types of functions are:
• Aggregate Functions
• Scalar functions
Aggregate functions
Aggregate functions operate against a collection of values, but return a single value.
Note: If used among many other expressions in the item list of a SELECT statement, the SELECT must have a GROUP BY clause!!
"Persons" table (used in most examples)
|Name |Age |
|Hansen, Ola |34 |
|Svendson, Tove |45 |
|Pettersen, Kari |19 |
Aggregate functions in MS Access
|Function |Description |
|AVG(column) |Returns the average value of a column |
|COUNT(column) |Returns the number of rows (without a NULL value) of a column |
|COUNT(*) |Returns the number of selected rows |
|FIRST(column) |Returns the value of the first record in a specified field |
|LAST(column) |Returns the value of the last record in a specified field |
|MAX(column) |Returns the highest value of a column |
|MIN(column) |Returns the lowest value of a column |
|STDEV(column) | |
|STDEVP(column) | |
|SUM(column) |Returns the total sum of a column |
|VAR(column) | |
|VARP(column) | |
Aggregate functions in SQL Server
|Function |Description |
|AVG(column) |Returns the average value of a column |
|BINARY_CHECKSUM | |
|CHECKSUM | |
|CHECKSUM_AGG | |
|COUNT(column) |Returns the number of rows (without a NULL value) of a column |
|COUNT(*) |Returns the number of selected rows |
|COUNT(DISTINCT column) |Returns the number of distinct results |
|FIRST(column) |Returns the value of the first record in a specified field (not supported in |
| |SQLServer2K) |
|LAST(column) |Returns the value of the last record in a specified field (not supported in |
| |SQLServer2K) |
|MAX(column) |Returns the highest value of a column |
|MIN(column) |Returns the lowest value of a column |
|STDEV(column) | |
|STDEVP(column) | |
|SUM(column) |Returns the total sum of a column |
|VAR(column) | |
|VARP(column) | |
Scalar functions
Scalar functions operate against a single value, and return a single value based on the input value.
Useful Scalar Functions in MS Access
|Function |Description |
|UCASE(c) |Converts a field to upper case |
|LCASE(c) |Converts a field to lower case |
|MID(c,start[,end]) |Extract characters from a text field |
|LEN(c) |Returns the length of a text field |
|INSTR(c) |Returns the numeric position of a named character within a text field |
|LEFT(c,number_of_char) |Return the left part of a text field requested |
|RIGHT(c,number_of_char) |Return the right part of a text field requested |
|ROUND(c,decimals) |Rounds a numeric field to the number of decimals specified |
|MOD(x,y) |Returns the remainder of a division operation |
|NOW() |Returns the current system date |
|FORMAT(c,format) |Changes the way a field is displayed |
|DATEDIFF(d,date1,date2) |Used to perform date calculations |
SQL GROUP BY and HAVING
Aggregate functions (like SUM) often need an added GROUP BY functionality.
GROUP BY...
GROUP BY... was added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called, and without the GROUP BY function it was impossible to find the sum for each individual group of column values.
The syntax for the GROUP BY function is:
|SELECT column,SUM(column) FROM table GROUP BY column |
GROUP BY Example
This "Sales" Table:
|Company |Amount |
|W3Schools |5500 |
|IBM |4500 |
|W3Schools |7100 |
And This SQL:
|SELECT Company, SUM(Amount) FROM Sales |
Returns this result:
|Company |SUM(Amount) |
|W3Schools |17100 |
|IBM |17100 |
|W3Schools |17100 |
The above code is invalid because the column returned is not part of an aggregate. A GROUP BY clause will solve this problem:
|SELECT Company,SUM(Amount) FROM Sales |
|GROUP BY Company |
Returns this result:
|Company |SUM(Amount) |
|W3Schools |12600 |
|IBM |4500 |
HAVING...
HAVING... was added to SQL because the WHERE keyword could not be used against aggregate functions (like SUM), and without HAVING... it would be impossible to test for result conditions.
The syntax for the HAVING function is:
|SELECT column,SUM(column) FROM table |
|GROUP BY column |
|HAVING SUM(column) condition value |
This "Sales" Table:
|Company |Amount |
|W3Schools |5500 |
|IBM |4500 |
|W3Schools |7100 |
This SQL:
|SELECT Company,SUM(Amount) FROM Sales |
|GROUP BY Company |
|HAVING SUM(Amount)>10000 |
Returns this result
|Company |SUM(Amount) |
|W3Schools |12600 |
SQL The SELECT INTO Statement
The SELECT INTO Statement
The SELECT INTO statement is most often used to create backup copies of tables or for archiving records.
Syntax
|SELECT column_name(s) INTO newtable [IN externaldatabase] |
|FROM source |
Make a Backup Copy
The following example makes a backup copy of the "Persons" table:
|SELECT * INTO Persons_backup |
|FROM Persons |
The IN clause can be used to copy tables into another database:
|SELECT Persons.* INTO Persons IN 'Backup.mdb' |
|FROM Persons |
If you only want to copy a few fields, you can do so by listing them after the SELECT statement:
|SELECT LastName,FirstName INTO Persons_backup |
|FROM Persons |
You can also add a WHERE clause. The following example creates a "Persons_backup" table with two columns (FirstName and LastName) by extracting the persons who lives in "Sandnes" from the "Persons" table:
|SELECT LastName,Firstname INTO Persons_backup |
|FROM Persons |
|WHERE City='Sandnes' |
Selecting data from more than one table is also possible. The following example creates a new table "Empl_Ord_backup" that contains data from the two tables Employees and Orders:
|SELECT Employees.Name,Orders.Product |
|INTO Empl_Ord_backup |
|FROM Employees |
|INNER JOIN Orders |
|ON Employees.Employee_ID=Orders.Employee_ID |
SQL The CREATE VIEW Statement
A view is a virtual table based on the result-set of a SELECT statement.
What is a View?
In SQL, a VIEW is a virtual table based on the result-set of a SELECT statement.
A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database. You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from a single table.
Note: The database design and structure will NOT be affected by the functions, where, or join statements in a view.
Syntax
|CREATE VIEW view_name AS |
|SELECT column_name(s) |
|FROM table_name |
|WHERE condition |
Note: The database does not store the view data! The database engine recreates the data, using the view's SELECT statement, every time a user queries a view.
Using Views
A view could be used from inside a query, a stored procedure, or from inside another view. By adding functions, joins, etc., to a view, it allows you to present exactly the data you want to the user.
The sample database Northwind has some views installed by default. The view "Current Product List" lists all active products (products that are not discontinued) from the Products table. The view is created with the following SQL:
|CREATE VIEW [Current Product List] AS |
|SELECT ProductID,ProductName |
|FROM Products |
|WHERE Discontinued=No |
We can query the view above as follows:
|SELECT * FROM [Current Product List] |
Another view from the Northwind sample database selects every product in the Products table that has a unit price that is higher than the average unit price:
|CREATE VIEW [Products Above Average Price] AS |
|SELECT ProductName,UnitPrice |
|FROM Products |
|WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products) |
We can query the view above as follows:
|SELECT * FROM [Products Above Average Price] |
Another example view from the Northwind database calculates the total sale for each category in 1997. Note that this view select its data from another view called "Product Sales for 1997":
|CREATE VIEW [Category Sales For 1997] AS |
|SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales |
|FROM [Product Sales for 1997] |
|GROUP BY CategoryName |
We can query the view above as follows:
|SELECT * FROM [Category Sales For 1997] |
We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":
|SELECT * FROM [Category Sales For 1997] |
|WHERE CategoryName='Beverages' |
SQL Quick Reference
SQL Quick Reference from W3Schools. Print it, and fold it in your pocket.
SQL Syntax
|Statement |Syntax |
|AND / OR |SELECT column_name(s) |
| |FROM table_name |
| |WHERE condition |
| |AND|OR condition |
|ALTER TABLE (add column) |ALTER TABLE table_name |
| |ADD column_name datatype |
|ALTER TABLE (drop column) |ALTER TABLE table_name |
| |DROP COLUMN column_name |
|AS (alias for column) |SELECT column_name AS column_alias |
| |FROM table_name |
|AS (alias for table) |SELECT column_name |
| |FROM table_name AS table_alias |
|BETWEEN |SELECT column_name(s) |
| |FROM table_name |
| |WHERE column_name |
| |BETWEEN value1 AND value2 |
|CREATE DATABASE |CREATE DATABASE database_name |
|CREATE INDEX |CREATE INDEX index_name |
| |ON table_name (column_name) |
|CREATE TABLE |CREATE TABLE table_name |
| |( |
| |column_name1 data_type, |
| |column_name2 data_type, |
| |....... |
| |) |
|CREATE UNIQUE INDEX |CREATE UNIQUE INDEX index_name |
| |ON table_name (column_name) |
|CREATE VIEW |CREATE VIEW view_name AS |
| |SELECT column_name(s) |
| |FROM table_name |
| |WHERE condition |
|DELETE FROM |DELETE FROM table_name |
| |(Note: Deletes the entire table!!) |
| |or |
| |DELETE FROM table_name |
| |WHERE condition |
|DROP DATABASE |DROP DATABASE database_name |
|DROP INDEX |DROP INDEX table_name.index_name |
|DROP TABLE |DROP TABLE table_name |
|GROUP BY |SELECT column_name1,SUM(column_name2) |
| |FROM table_name |
| |GROUP BY column_name1 |
|HAVING |SELECT column_name1,SUM(column_name2) |
| |FROM table_name |
| |GROUP BY column_name1 |
| |HAVING SUM(column_name2) condition value |
|IN |SELECT column_name(s) |
| |FROM table_name |
| |WHERE column_name |
| |IN (value1,value2,..) |
|INSERT INTO |INSERT INTO table_name |
| |VALUES (value1, value2,....) |
| |or |
| |INSERT INTO table_name |
| |(column_name1, column_name2,...) |
| |VALUES (value1, value2,....) |
|LIKE |SELECT column_name(s) |
| |FROM table_name |
| |WHERE column_name |
| |LIKE pattern |
|ORDER BY |SELECT column_name(s) |
| |FROM table_name |
| |ORDER BY column_name [ASC|DESC] |
|SELECT |SELECT column_name(s) |
| |FROM table_name |
|SELECT * |SELECT * |
| |FROM table_name |
|SELECT DISTINCT |SELECT DISTINCT column_name(s) |
| |FROM table_name |
|SELECT INTO |SELECT * |
|(used to create backup copies of tables) |INTO new_table_name |
| |FROM original_table_name |
| |or |
| |SELECT column_name(s) |
| |INTO new_table_name |
| |FROM original_table_name |
|TRUNCATE TABLE |TRUNCATE TABLE table_name |
|(deletes only the data inside the table) | |
|UPDATE |UPDATE table_name |
| |SET column_name=new_value |
| |[, column_name=new_value] |
| |WHERE column_name=some_value |
|WHERE |SELECT column_name(s) |
| |FROM table_name |
| |WHERE condition |
Source :
Query Examples in Microsoft Access
Structured Query Language is a programming language developed within IBM corporation during the 1970's. This language is designed to provide retrieval and update of information stored in Relational Database Management Systems (RDBMS).
Oracle corporation has had great influence on expanding the functionality of SQL during the 1980's through the 1990's. SQL differs from other programming languages such as Basic & Fortran in that a user will identify which data is to be retrieve/updated by field name and table name and it is up to a SQL database engine to determine how to find information based on the table relationships originally defined by a database designer.
[pic][pic]
Microsoft Access query language is a fairly robust subset of the full SQL language. Access SQL query examples contained in this section of our website will help the non-professional programmer to hurdle stumbling blocks often encountered when creating a query. These query examples will provide 'how to' programming for several situations when you may need to get into the Access SQL script editor to customize your query.
Access Union Query Example
Any/All as a Choice in a Combo Box
This MS Access union query example shows how to create a combo box All choice item. This technique is often used for selecting specific (or All) records for a report.
The Union Query is used in this example and is a very powerful feature of the SQL language. We assume you have read our Microsoft Access database design recommendations.
[pic]
Note: Putting a blank letter before the 'A' in All makes this combo choice All choice sort to the top. Warning: General warning about union queries - Never try to use aggregate functions (avg, count, sum, etc.) in a union query. It will drop out duplicate groups of records (in Access at least ).
Filter Report Records via Form Field
Use Form Fields as Parameters in a Query
In this filter report records via form field example we use the Employee_ID selected in filter report example to select employee records for a report. Many users and developers would use filters on the report to accomplish this task.
Filters are ok to use but putting these restrictions in the SQL query for the report usually runs faster. This example assumes that you have a combo box (named: Emp_Combo), as defined in a previous example, in a form called F_Emp.
[pic][pic]
Filter Report Records Using Form Field Parameter Example:
Select M_Employees.Name, M_Employees.Emp_Number, M_Employees.Address
From M_Employees
Where
Forms!F_Emp!Emp_Combo=0 or Forms!F_Emp!Emp_Combo = M_Employees.Employee_ID;
This SQL query will select a specific employee or All employees for the report based on the Emp_Combo in the parameter form (F_Emp). Note: You could have several combo boxes on the parameter form which would allow the user to select Employee records based on a combination of several fields. This filter report records method provides a very powerful reporting feature. (Warning: Access may attempt to re-write your 'where' clause making it non-functioning).
Warning: If you ever want to upsize this Microsoft Access database to SQL Server then you should not refer to form fields within the query. SQL Server can't deal with these references
Dynamic Order By Query Clause Example
Immediate If (IIF) Order By Clause in SQL Queries
Create a dynamic order by clause using the iif (immediate if) statement to change the 'Order By' statement in a query based on user input. There are many instances where you would like to sort a form or report on different fields depending on a user's selection.
[pic][pic]
The following is an SQL example of an Access dynamic Order By clause. This example assumes you have a form ('F_Emp') with an Option Group ('Sort_Option') with two possible choices. Option 1 is for sorting by employee name and option 2 is for sorting by employee number. Variable sorting can be accomplished in the Report however it is much more efficient to put this in the SQL code.
Select M_Employees.Name, M_Employees.Emp_Number, M_Employees.Address
From M_Employees
Order by
IIf(Forms!F_Emp!Sort_Option=1, M_Employees.Emp_Name, M_Employees.Emp_Number);
An alternative form of the Order By statement is to use ordinals. Ordinals are numbers that refer to the fields in the select clause. Example: ordinal 1 refers to M_Employees.Name, and ordinal 2 refers to M_Employees.Emp_Number. IIf(Forms!F_Emp!Sort_Option=1, 1, 2); This form of the IIf clause is very useful when you need to nest IIf clauses because of more than two sort choices.
The immediate if (IIF) statement can also be used to create dynamic where query statements
Choose Function in Microsoft Access
The choose function is similar to the Decode function in Oracle. Assume the same setup as the Dynamic Order By Clause example but add a 3rd option for sorting by Address.
See below how the choose function expands your capabilities:
[pic][pic]
Select M_Employees.Name, M_Employees.Emp_Number,
M_Employees.Address
From M_Employees
Order by
Choose(Val(Forms!F_Emp!Sort_Option),M_Employees.Emp_Name,
M_Employees.Emp_Number, M_Employees.Address);
Note: Entry in Forms!F_Emp!Sort_Option must be an integer and it is best to force it to a number with the Val() function. You can have up to 29 options and you can use the Choose Function in each part of the query except the From clause.
The choose function can be used in each part of a query, except possible the from clause.
Access Bottom Up Query / Detail-Master Queries
Examples
There are times when you need to retrieve master records based on criteria in a detailed record. We will assume that you have two tables in a master detail relationship.
[pic][pic]
The master is M_Employees and the detail is M_Attendance. These are linked by the Employee_ID field. Our goal is to find the name and number of the employees that have missed more than 5 days of work.
Select M_Employees.Name, M_Employees.Emp_Number
From M_Employees
Where M_Employees.Employee_ID in
(Select Employee_ID from M_Attendance
Group By Employee_ID
Having Count(M_Attendance.Day_Missed) >= 5);
Hopefully you can see how to parameterize this query to select a variable number of missed days. Or, you could use the Choose command to select the count of a different field.
Parameter Query in Access
Parameter query example is an extension of the bottom-up query example. The form field is used in the query to determine how far to look back in the M_Attendance table for missed work days. Also, you are running this parameter query from a form called F_Emp_Report.
Access Parameter Query Code Example:
Select M_Employees.Name, M_Employees.Emp_Number
From M_Employees
Where M_Employees.Employee_ID in
(Select Employee_ID from M_Attendance
Where M_Attendance.Attendance_Date >= Forms!F_Emp_Report!Start_Date
Group By Employee_ID
Having Count(M_Attendance.Day_Missed) >= 5);
Note: If you get an Microsoft Access error saying that the form or field Forms![F_Emp_Report]!Start_Date (the parameter) cannot be found then you need to get the latest update for Access 2000, Version 9.0.4402 SR-1. Referencing a form when using an aggregate function (like count, max, min, avg) produces this error when using form fields for parameter queries.
Update Table Data Fields
In this Update Query example we want to update each employee's salary by 10%. There are at least two ways to do this query.
Example c) is supposed to work but I get errors in Access. It is called a correlated subquery - if you can tell me what is wrong with it I'd really appreciate it.
[pic][pic]
Same Table Update Query Code:
a) Update M_Employees as A INNER JOIN M_Employees as B ON A.Employee_ID = B.Employee_ID SET A.Salary = B.Salary*1.1
b) Update M_Employees as A, M_Employees as B Set A.Salary=B.Salary * 1.1
Where A.Employee_ID=B.Employee_ID
c) Update M_Employees as A set A.Salary = (Select Salary * 1.1 from M_Employees Where M_Employees.Employee_ID = A.Employee_ID)
Note: I have added a another feature to this example - Aliases. Aliases are where you use 'as' to establish a short nickname for a table or a field. This helps when you have long table names or are trying to do a correlated subquery.
Form Field as Combo Box Filter
We are working with an MS Access form for Employees and we need to define a Employee's Supervisor for each Employee. However, Employee Supervisors only advise Employees in specific departments.
So, based upon the Employee's department a different list of potential Supervisors will be displayed in the combo box. The form, F_Employees, his three fields: Employee_Name, Departement_Combo, and Supervisor_Combo. The following is the row source for the Supervisor_Combo:
Select Supervisor_ID, Supervisor_Name
From L_Supervisors
Where L_Supervisors.Departement_ID = Forms![F_Employees]![Departement_ID];
[pic][pic]
Note that the Departement_Combo is made up of two fields: Departement_ID (not visible) and Major. Supervisor_Combo also has two fields: Departement_ID (not visible) and Major.
Now this should work ok and it may appear to work on a one record form or on the first record of a multiple record form. However, Access is stupid and will not automatically check to see if the Employee_Major field has changed or if we have moved to a new record in a multi-record form. Re-quering every combo or listbox on a form on every new record could slowdown the form a lot. Therefore, to get by this you need to do two things:
1) In the After Update event on the Departement_Combo field of the F_Employees form add the command:
Me.Supervisor_Combo.Requery
2) In the On Current event of a multi-record
form add the same command as in 1).
More form field as drop down list box source filter examples can be found on our Combo Box Examples home page.
Pivot Query How To
The pesky pivot query. For this Access coding example let us assume we have a table with the following layout:
[pic]
Our goal is to total sales (Amount) by month (Sale_Date) and Department with months as column headings and departments as row headings.
[pic][pic]Access provides a pivot query wizard which, once you get some practice you'll find it as good place to start. I usually have to go into the SQL editor and tweak the code a little. Here are the SQL statements that will produce the output we want:
TRANSFORM Sum([M_Sales].[Amount]) AS SumOfAmount
SELECT [M_Sales].[Department]
FROM M_Sales
GROUP BY [M_Sales].[Department]
PIVOT Format([M_Sales].[Sale_date],"mmm") In ("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec");
The key points of the query are:
1) The query's TRANSFORM statement contains the values that will be summed and displayed as the results of the query.
2) The SELECT statement will contain the Row Headings
3) The GROUP BY is just what it says 4) The PIVOT statement will create the column headings and may be thought of as a Horizontal Group By. Here are the results of the query:
[pic]
Warning about upsize to SQL Server - Access pivot query is not support. There are workarounds which are similar to the functioning of the Histogram Example (#16).
Select Top Query
Select Top n Records Predicate Example
Have you ever had the need to get the 3rd record from a table using a query only?
Well, in case you want to know how to do it here is the solution:
1) Create a query (Query1) to get the top three records. We are interested in field called Submit_Date from a table called M_Revisions:
Select Top 3 M_Revisions.Submit_Date from M_Revisions
Order by M_Revisions.Submit_Date;
2) Use Query1 as input to a new query:
Select Top 1 Query1.Submit_Date from Query1
Order by Query1.Submit_Date DESC;
Now you have the 3rd submit date. Note that sorting the Query1 records in descending order makes the 3rd record go to the top.
Date/Time Variables in SQL Query
Access date/time query example is the topic of this discussion. Many types of data change over time some examples include work pay, hourly consulting rates, part costs, etc.
This is an example of calculating worker pay as their pay rate changes over date/time. You can see below there are two tables.
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• The M_Employees table contains the hours worked for each date/time.
• The M_Employee_PayRate table contains the worker's salary history over time.
In the image below, queries 1 and 2 are the same query but I made a copy so I could show one in design view and the other after it runs.
The key to this Access date/time query is to have accurate pay rates defined by the Start and Stop dates. Note that the current pay rate does not have a 'Stop_Date'. The query substitutes today's date for the null stop date.
[pic]
Dynamic Table Links in Microsoft Access
Here's a neat trick which allows you to forego the need for permanently link tables between a front end and back end Access database setup.
Access dynamic table link works by using IN followed by a path and database name will retrieve the data dynamically. If you put this query in VBA (password protected) then the user will not know where the data is coming from. You won't have to worry about locking up the database objects which makes database updates/maintenance a bit cumbersome.
Access dynamic table link code:
SELECT Customer_ID, Sales_Rep_ID
FROM m_customer_orders IN 'p:\database\shoe_orders_data.mdb';
Another great way to use the dynamic table links technique is when you have an 'active database' and an 'archive database' the user can dynamically switch between the two with an option on a form and a little VBA to create the alternate path to the data using dynamic table links.
Future examples will show how to create a temporary Access database and then create temporary tables in the database for processing and report generation - this technique eliminates front end database bloat.
Access Choose Function Example
Have you ever had to create several reports that were almost identical but had different order for the columns and a different sort order? Many of these reports can be done in one intelligent report and query combination with the Access SQL Command: Choose function.
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Access Choose function query example setup: We have a form called Form1. On Form1 are two combo boxes (Field1_Combo, Field2_Combo), and a 'Preview Report' button:
[pic]
Both combo boxes have the same Row Source. Note that there are two columns in the row source; the first is a number (1, 2) and the second is the name of the field to be included in the report and sorted by.
The query looks like this:
[pic] [pic]
Note that we will always retrieve the employee's Pay_Rate field, however this could be variable as well. You'll also need to see the SQL code for the query to see how the Order By clause works for this Access choose command example:
SELECT
Choose(Val([forms]![form1]![field1_combo]),[SSN],[Employee_No]) AS Field1,
Choose(Val([forms]![form1]![field2_combo]),[SSN],[Employee_No]) AS Field2,
M_Emp_Pay.Pay_Rate
FROM M_Emp_Pay
ORDER BY
Choose(Val([forms]![form1]![field1_combo]),[SSN],[Employee_No]),
Choose(Val([forms]![form1]![field2_combo]),[SSN],[Employee_No]);
Here's the resulting report:
[pic]
The second trick is how I got the column headings to come out correctly... All that's required is one line of VBA code for each label.. but before I show that here is the design view of the report:>
[pic]
Ok, let's see how the VBA code assigns the correct values to the labels:
Option Compare Database
Private Sub PageHeaderSection_Format(Cancel As Integer, FormatCount As Integer)
Me.Field1_Label.Caption = Replace(Forms![form1]![Field1_Combo].Column(1), "_", " ")
Me.Field2_Label.Caption = Replace(Forms![form1]![Field2_Combo].Column(1), "_", " ")
End Sub
Don't be confused by the Replace command... all that does is get rid of underscore character in the field name (Employee_No to Employee No).
Using the Access Choose function query can be extended to numerous fields and could probably be used for aggregate functions too, but I haven't tried that yet. The alternative to using the SQL Choose command is to write many lines of VBA code in the report or behind the parameter form (Form1).
You can also use the choose function to pass parameters to an Access Query.
Access Scalar Query
Access scalar query allows you to do in one SQL statement what you are used to doing in two or more queries. This function allows retrieval of single values from a table, usually aggregate functions, from within the from clause.
Get individual values while at the same time getting max, min, avg, etc values from the same source without having to use the Group By clause... this greatly simplifies query design. Here's the setup for our scalar query example:
[pic][pic]
[pic]
Our goal is the retrieve SSN, Pay_Rate, Max Pay_Rate, Min Pay_Rate, and calculate each employees' percent of maximum pay rate.
See the following SQL statement:
[pic]
Don't bother trying to create this in the design grid. You must get into SQL view. The key point of the query is:
[select max(pay_rate) as Max_R from m_emp_pay]. as Q_Max There are two aliases in this subquery - Max_R for the field and Q_Max for the source name. See how these aliases are immediately used in the Select clause of the query. No group by required!! Therefore you get the individual pay rates for employees while, at the same time, retrieving min, max, and most importantly the percent of max.
See the results of the query below:
[pic]
The main restriction with scalar SQL statement is that the function can only return a single value, although you can have multiple subqueries in one main query.
Histogram Query
Access Histogram query example - you'll be surprised at how easy it is to do the calculations for a seemingly complex statistical measure using a single Access query. In this example we have customers who purchased items from a store.
We want to know the distribution of customer purchases grouped by customer's age. To start, we have a table called M_Customer_Purchases (in reality this would probably be a query based on a Customer table and a Purchases table).
[pic][pic]
The table has 2 fields (the purchase_date field is really not required for this example).
Age
Purchase_Date
The following sql statement will group the ages (purchases) into 6 categories based on the customer's age. You can see the bracketing of the age in the immediate if statement (iif) - this created the Access histogram values.
SELECT
Sum(IIf([Age]=18 and [Age]=30 and [Age]=40 and [Age]=50 and [Age]=60,1,0)) AS Group6
FROM M_Customer_Purchases;
The following is the result of the Access example query. You could easy pass this query to a bar chart to display the results graphically.
[pic]
Note the trick is the 1, 0 in the iif statement - when the age matches the age range in the iif statement then the result is one, otherwise the result is 0, thereby providing a way to SUM the results.
You should be able to see how you can bracket results for all kinds of data. You could bracket date ranges to simulate a crosstab query (pivot query). One advantage of using the method in this example over crosstabs is that you can pass query parameters to the query from a form field.
Calculate Running Sum Recordset Example
Here is a fairly simple way to calculate a running sum using a DAO Recordset operation. To begin, we have created a temporary table with Absence Dates, and Substitute Teach ID (SubID) - this table was created via a previous query not shown in this example.
Our goal is to determine running sum of substitute records based on absence date (teacher was absent and therefore a sub worked in this day). In the example we used rst!day_count rather than just counting records because the sub can work either 1/2 days or full days (1). Here is the VBA code:
[pic][pic]
Private Sub Sum_Button_Click()
On Error GoTo Err_Sum_Button_Click
Dim rst As DAO.Recordset
Dim db As DAO.Database
Dim hold_subid As Long
Dim hold_day_Count As Long
Dim sqltext As String
Dim wksp As DAO.Workspace
Set db = CurrentDb
'
'calculate running sum of days worked.
'
Set wksp = DBEngine.Workspaces(0)
wksp.BeginTrans
Set rst = db.OpenRecordset("Select * from t_Sub_Pay order by subid,absencedate")
rst.MoveFirst
hold_subid = rst!SubID
hold_day_Count = 0
'
Do While Not rst.EOF
If hold_subid rst!SubID Then
' note that we reset the counter when a new sub teacher is encountered
' new sub teacher
hold_day_Count = rst!Day_Count
hold_subid = rst!SubID
Else
hold_day_Count = hold_day_Count + rst!Day_Count
End If
rst.Edit
rst!running_sum = hold_day_Count ' calculate runing sum here
rst.Update
rst.MoveNext
Loop
mitTrans
rst.Close
set rst = Nothing
wksp.close
exit sub
Err_Sum_Button_Click:
wksp.rollback ' cancel transactions if there is an error
rst.close
set rst=nothing
wksp.close
resume quit_it
quit_it:
End Sub
Note that this example also uses Transaction Processing - in this case the time to do the running sum processing was reduce by about 75%.
See some additional queries in Access Visual Basic.
SQL Predicates - Optional Value After the Select Keyword
SQL Predicates are simple but important to understand for full use of the query programming language.
There are five predicate functions:
• Select All
• Select Distinct
• Select Distinctrow
• Select Top
• Select Top Percent
[pic][pic]
The ALL command is the default when you use a select statement:
Select * from Employees
is equivalent to entering
Select All * from Employees
The Distinct command is often confused with the distinctrow keyword. Here is an example of the difference:
Select Distinctrow Last_Name from Employees
This query won't necessarily retrieve a distinct list of employee last names. If there are duplicates last names and ANY other field has different data between the two duplicate name records then you will get both records even though they have the same last name. Whereas:
Select Distinct Last_Name from Employees
Will retrieve a unique list of employee last names because the Distinct command only looks at the fields you are returning in the query. I have never had a reason to use the Distinctrow function because our tables never have duplicate rows.
Select TOP (aka Top Values/Top Values) is explained in an example page: Select Top 10 Records, but we will review it here combined with the PERCENT option:
Select Top 10 Last_Name from Employees
Order By Age desc
Using the Top 10 example tells the query engine to return 10 records, in this case it will be the 10 oldest employees. Here is the percent option:
Select Top 10 Percent Last_Name from Employees Order By Age desc
In this case, if you had 1000 employees in the Employee table you would retrieve 100 records containing a list of the 100 oldest employees.
Now you know all about the use of predicates in the Microsoft Access programming language.
Update Master Record Based on Detail Records
Bottom-up Query Examples - Detail Record Criteria Selects Master Records
In this example let us assume we have two tables in a master/detail relationship. The main table contains Tasks which much be completed.
The detail table contains one or more Action Items for each task. The two tables are linked by Task_ID. All Action Items must be completed before the Task is complete. We want to mark the task as done when all the action items have been completed. We have an AfterUpdate trigger associated with the Completed field in the Action Items form. Here is the code to update the master table (Tasks) when all items are completed for the task.
[pic][pic]
Private Sub Action_Complete_AfterUpdate()
If Me.Action_Complete=true then
DoCmd.RunSQL ("Update Tasks set Task_Complete=True " & _
" Where Tasks.Task_ID in " & _
" (Select Task_ID from Action_Items where Task_ID=" & Me.Task_ID & _
" Having Max(Action_Items.Action_Completed)=-1 " & _
" Group By Task_ID)")
Else
DoCmd.RunSql ("Update Tasks set Task_Complete=False " & _
" where Task_ID=" & Me.Task_ID)
End If
End Sub
Ok... this is more of an SQL example than a visual basic example. But this is a perfect example of creating a very simple solution to a potentially very complicated task. The key to creating highly maintainable Access databases is to have a consultant who knows both SQL programming and visual basic programming.
Here is the trick to this query... If all the Action_Completed Yes/No fields are true (-1) then the Max of them will be -1. If any are not true (0) then the Max will be 0.
You should have indexes on all table fields mentioned in the SQL statement.
Crosstab Query Added Trick
Here is a slight twist to a simple MS Access crosstab query that may not seem obvious to you.
In this example we want only the totals by month and we don't care about department.
[pic][pic]
TRANSFORM Sum([M_Sales].[Amount]) AS SumOfAmount
SELECT "Total" AS Total
FROM M_Sales
GROUP BY "Total"
PIVOT Format([M_Sales].[Sale_date],"mmm") In ("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec");
We need to select something so I put in the constant 'Total' so that the pivot query will run. We don't want to see 'Total' in the first column so we will hide this column. Here are the results:
[pic]
Tip: If you need to do some restriction to select only certain records then create that query first and use it as the input to the pivot query. It will make things simpler for you.
Success for You All
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