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
- If the expression has NULL value, the condition returns TRUE result
- 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.