Introduction to SQL Server Check Constraints

Check Constraints play a very important role in SQL Server
as it allows data validation rules to be imposed at the SQL Server instance level itself instead of writing rules for each application. Check Constraint enforces Domain Integrity by
restricting the values that can be inserted into columns. Domain Integrity ensures that only a valid range of
values are allowed to be stored in a field.

To understand the Check constraint on a table
let’s take a practical example of a student table as shown below.

Create table student

Student_id int primary key,
Student_first_name varchar(50),
Student_last_name varchar(50),
Sex varchar(1),
Age int,

Now, create a Check Constraint on the column named Sex
which can have only 2 values named M and F (i.e Male and Female).

ALTER TABLE dbo.student
CHECK (Sex in ('M','F'));

Once the above Check Constraint is created, SQL Server will
understand that there are only two valid values for Sex  column – namely ‘M’
and ‘F’ . No other value would be acceptable.

Let’s test this by attempting to insert records into the student table:

insert student

select 1,'ABC','PQR','L',12

This will cause the below error to be thrown :

Msg 547, Level 16, State 0, Line 2 

The  INSERT statement conflicted with the CHECK constraint
"CC_student_Sex". The conflict occurred in database
"student", table "dbo.student", column 'Sex'.

The statement has been terminated.

This was caused as Sex has been specified as L
which is not a valid value as compared to the Check Constraint definition.

Now lets try to insert a valid set of data using the below

insert student

select 1,'ABC','PQR','M',12

The above INSERT operation would be successful as it
meets the Check Constraint definition.

Now, we can place a Check Constraint on the Age column using the below T-SQL.

ALTER TABLE dbo.student


CHECK (Age>=3);

The above Check Constraint rule means that a student
age needs to be greater than or equal to 3 years.

Now lets this constraint by attempting to insert data using the below T-SQL.

insert student

select 2,'ABC','PQR','M',1

This will fail as the value of age is
less than or equal to 3 which is not acceptable as per the Check Constraint, retrying this will a value of 4 will succeed.

If there are multiple Check
Constraints they all must be satisfied for data to be successfully added.

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