MS SQL Server: DISTINCT Clause in SQL Server

In SQL Server (Transact-SQL), the DISTINCT clause helps remove duplicates in the result set. The DISTINCT clause can only be used in the SELECT statement.

DISTINCT clause syntax

 SELECT DISTRINCT 'bi?u th?c' 
 FROM 'b?ng' 
 [WHERE '?i?u ki?n']; 

Variable name or variable value

'expression'

The column or calculated value you want to retrieve

'table'

Table used to retrieve records. Must have at least 1 table in the FROM clause.

WHERE 'condition'

Option. Conditions that the record must meet to be selected.

Note

• When the DISTINCT clause has only one expression, the query returns unique values for that expression.
• When the DISTINCT clause has more than 1 expression, the query returns the unique combination of expressions.

In SQL Server, the DISTINCT clause does not ignore the NULL value. So when using this clause in the command, the returned result will have a unique NULL value.

Example - 1 expression

The simplest example with the DISTINCT clause in SQL Server has only one expression.

 SELECT DISTINCT ho 
 FROM nhanvien 
 WHERE nhanvien_id >= 50; 

This example will return all employee surname values from the table with the table number greater than or equal to 50.

As an example, - multiple expressions

 SELECT DISTINCT ten, ho 
 FROM nhanvien 
 WHERE nhanvien_id >= 50 
 ORDER BY ho;