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DISTINCT clause in SQL Server

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

DISTINCT clause syntax

 SELECT DISTRINCT 'biểu thức' 
 FROM 'bảng' 
 [WHERE 'điều kiện'];

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

  1. When the DISTINCT clause has only one expression, the query returns unique values ​​for that expression.
  2. 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.

For example - multiple expressions

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

In this example, the returned result will be the combination of the first and last name from the table when the value is greater than or equal to 50. The result is sorted in ascending order of the employee.

In this case, DISTINCT applies to each information field behind the DISTINCT keyword, so it will create different name combinations - they are different.

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Next article: IN conditions in SQL Server

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