SQL DISTINCT clause

SQL DISTINCT clause

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 doesnt 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

Lets 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

Lets 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

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