Check Constraint in MS SQL Server
Check Constraint :
It is used alongside relational operators to check whether a value satisfies the condition or not (boolean). If the condition is satisfied, the boolean expression sets to True otherwise False. The check constraint does not have a specific syntax. It is used along with the create table syntax.
Syntax :
Create table Marks name varchar2(30), rollnumber number primary key, marks int check (marks<=75)
A table named Student is created along with the condition that marks must not be greater than 75. A user inserts a few values as shown below –
Table – Marks
Name | Rollnumber | Marks |
---|---|---|
Aisha | 111 | 60 |
Naina | 112 | 73 |
The values are inserted as per the conditions mentioned in the create table syntax. The user tries inserting a few more values yet errors occur as shown below –
Example-1:
Insert into Student values('Maya', '117', '80')
Output –
The value results in an error as the value is greater than 75.
Example-2:
Insert into Student values('Maya' '111', '74')
Output –
An error is displayed. This is due to the primary key used for rollnumber. Primary key forbids the use of duplicates in a table.
Check constraint in case of NULL :
Insert into Student values('Riya', '112', 'NULL')
Output –
In SQL, NULL is used incase of unknown value. Therefore it is considered as False.
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