WHAT ARE THE CONSTRAINTS IN SQL?

CONSTRAINTS are used to create rules for data in a table to limit the type of data that can be stored into a table, Constraints can be created when the table is created with the CREATE TABLE statement , or after the table is created with the ALTER TABLE statement in SQL.

Syntax of CONSTRAINTS in SQL-

CREATE TABLE <table_name>(<column_name1><datatype><constraint>,………………………<column_nameN><datatype> <constraint>);

CONSTRAINTS	Usage
NOT NULL	To ensure that column can’t have a NULL value
UNIQUE	To ensure that all vales in a column are different
PRIMARY KEY	To ensure unique identification of each row in a table.It is a combinaton of NOT NULL & UNIQUE .
FOREIGN KEY	To prevent action that would destroy links between tables.
CHECK	To ensure that the values in a column meet a specific condition.
DEFAULT	To set a default value for a colum if no value is specified.
CREATE INDEX	To create and retrieve data from the database quickly.