Constraints are some rules that enforce on the data to be enter into the database table. Basically constraints are used to restrict the type of data that can insert into a database table. Constraints can be defined in two ways:

seen from T1

seen from United Kingdom
seen from United States

seen from Belgium

seen from T1
seen from Belgium
seen from T1
seen from Belgium

seen from United States
seen from T1

seen from T1

seen from T1

seen from T1
seen from T1
seen from T1
seen from United States
seen from T1

seen from El Salvador
seen from T1
seen from United States
Constraints are some rules that enforce on the data to be enter into the database table. Basically constraints are used to restrict the type of data that can insert into a database table. Constraints can be defined in two ways: