SQL DISTINCT clause

Posted by : Glen Sajori Sabtu, 27 September 2014

Description

The SQL DISTINCT clause is used to remove duplicates from the result set of a SELECT statement.

Syntax

The syntax for the SQL DISTINCT clause is:

SELECT DISTINCT expressions
FROM tables
WHERE conditions;

Parameters or Arguments

expressions are the columns or calculations that you wish to retrieve.

tables are the tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

conditions are conditions that must be met for the records to be selected.

Note

When only one expression is provided in the DISTINCT clause, the query will return the unique values for that expression.
When more than one expression is provided in the DISTINCT clause, the query will retrieve unique combinations for the expressions listed.
In SQL, the DISTINCT clause doesn't ignore NULL values. So when using the DISTINCT clause in your SQL statement, your result set will include NULL as a distinct value.

Example - With Single field

Let's look at the simplest SQL DISTINCT query example. We can use the SQL DISTINCT clause to return a single field that removes the duplicates from the result set.

For example:

SELECT DISTINCT city
FROM suppliers;

This SQL DISTINCT example would return all unique city values from the suppliers table.

Example - With Multiple fields

Let's look at how you might use the SQL DISTINCT clause to remove duplicates from more than one field in your SQL SELECT statement.

For example:

SELECT DISTINCT city, state
FROM suppliers;

This SQL DISTINCT clause example would return each unique city and state combination. In this case, the DISTINCT applies to each field listed after the DISTINCT keyword.

Source : techonthenet.com