Why are use the SQL CHECK Constraint in the database at table creation?

Why are use the SQL CHECK Constraint in the database at table creation?

Last updated:9/22/2021 12:51:29 AM

1 Answers

Ashutosh Kumar Verma
Ashutosh Kumar Verma

SQL Check Constraint:

CHECK constraint is used to give a limit for the values to be passed on the column.

Data is inserted on the column according to the condition on which the CHECK constraint is given. A check constraint is used in SQL server allow you to specify condition on each row in a table.

CHECK constraints are column-level (single column) and table-level (multiple columns).

Check constraint is create two times, first when database table is created and other is alter table in exists table in database

1- SQL Check Constraint on table create:

The following SQL creates a Check constraint on the 'Age' column when the 'Student' table is created. The Check constraint ensures that the age of a Student must be 18, or older:

create table Student(

stuID int not null primary key,
stuName varchar(255),
stuCourse varchar(255),
stuAge int CHECK (stuAge>=18) );

Check constraint on multiple columns in a table

create table StudentRecord(

stuID int not null primary key,
stuName varchar(255),
stuCourse varchar(255),
stuAge int ,
stuMarks int ,
CONSTRAINT chk_StudentRecord CHECK (stuAge>=18 AND stuMarks>=40) );


2- SQL Check Constraint on created table:

The following SQL creates a Check Constraint on student Age in “Student” table which is created in database. Here the Check Constraint is used with SQL ALTER command. Ex- for single column in table,

ALTER TABLE Student

ADD CHECK (stuAge>=18);

For multiple columns of a table,

ALTER TABLE StudentRecord

ADD CONSTRAINT chk_Students CHECK (stuAge>=18 AND stuMarks>=40);

In the above Student table the records entered in which stuAge must be 18 or above and stuID is more than 3.

Drop Check Constraint:

 To Drop a created Check Constraint in table, use the following SQL statement,

ALTER TABLE Student

DROP CONSTRAINT chk_Student;

Other one is-

alter table StudentRecord

drop constraint chk_StudentRecord;


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