SQL CHECK

SQL CHECK constraint check specified specific condition, which must evaluate to true for constraint to be satisfied.

SQL CHECK constraint must be specified as a logical expression that evaluated specific condition either TRUE or FALSE.

SQL CHECK constraint define at column level that allow only specified values for a column and Whereas define at table level that allow specified values as well as use other column value for checking specific condition.

 CREATE TABLE student_info(
  no NUMBER(3) PRIMARY KEY,
  stu_code VARCHAR(10) CHECK (stu_code like 'pr%'),
  name VARCHAR(30) CHECK ( name = upper(name) ),
 city VARCHAR(30) CHECK (city IN ('LUcknow','DELHI','KANPUR'))
 scholarship NUMBER(5) CHECK (scholarship BETWEEN 500 AND 20000)
);

We are create new student_info table name with following check constraints:

Values inserted into stu_code column must be start with the lower letter 'pr'.
Values inserted into name column must be capitalize.
Values inserted into city column only allow lucknow,kanpur,delhi as valid legitimate values.
Values inserted into scholarship column between 500 and 20000.

ADD CHECK constraint (ALTER TABLE)

ALTER TABLE statement to add CHECK constraint in existing table column.

Syntax :
ALTER TABLE table_name
 ADD CONSTRAINT check_constraint_name CHECK (column_name condition);
ALTER TABLE student_info ADD CONSTRAINT check_name CHECK (name = upper(name));

DROP CHECK constraint (ALTER TABLE)

ALTER TABLE statement to drop CHECK constraint in existing table column.

Syntax :
ALTER TABLE table_name
 DROP CONSTRAINT check_constraint_name;