IS NULL condition in SQL Server

In SQL Server (Transact-SQL), the condition IS NULL is used to check the NULL value. A NULL value in a table is a value in an empty field, in other words, a field with no value.

Syntax condition IS NULL

  IS NULL 'expression' 

Variable name or variable value

expression

Values ​​to check if the value is NULL.

Note

1. If the expression has NULL value, the condition returns TRUE result
2. If the expression has no NULL value, the condition returns FALSE

For example - SELECT command

Let's look at the example condition NULL in the SELECT statement below.

 SELECT * 
 FROM nhanvien 
 WHERE ho IS NULL; 

This example will return all records in the table if the employee's last name is left blank - or called NULL.

Example - INSERT command

 INSERT INTO nhanvien 
 ((nhanvien_id, ho, ten) 
 SELECT nhanvien_id, ho, ten 
 FROM danhba 
 WHERE ten IS NULL; 

This command will fill in records from the list into the user table in the fields whose employee name is left blank.

Example - UPDATE command

 UPDATE nhanvien 
 SET ten = 'Unknown' 
 WHERE ten IS NULL; 

In this example, the records in the user table whose blank name value will be updated.

Example - DELETE command

 DELETE FROM nhanvien 
 WHERE ho IS NULL; 

This command will delete all records in the table if the value in the last name field is NULL.

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