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

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Ashutosh Kumar Verma · Answer

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

Ravi Vishwakarma

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