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.
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
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 –
Insert into Student values('Maya', '117', '80')
The value results in an error as the value is greater than 75.
Insert into Student values('Maya' '111', '74')
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')
In SQL, NULL is used incase of unknown value. Therefore it is considered as False.