Constraints are used to set the rules for all records in the table. If any constraints get violated then it can abort the action that caused it.
Constraints are defined while creating the database itself with the CREATE TABLE statement or even after the table is created once with the ALTER TABLE statement.
There are 5 major constraints are used in SQL, such as
NOT NULL : That indicates that the column must have some value and cannot be left NULL.
UNIQUE : This constraint is used to ensure that each row and column has a unique value and no value is being repeated in any other row or column.
PRIMARY KEY : This constraint is used in association with NOT NULL and UNIQUE constraints such as on one or the combination of more than one column to identify the particular record with a unique identity.
FOREIGN KEY : It is used to ensure the referential integrity of data in the table. It matches the value in one table with another using the PRIMARY KEY.
CHECK : It ensures whether the value in columns fulfills the specified condition.