16 – HOW TO CREATE TABLE CONSTRAINT
Integrity constraint-a rule that restricts the values in a database,
• A NOT NULL constraint prohibits a database value from being null,
• A unique constraint prohibits multiple rows from having the same value in the same column or combination of columns but allows some values to be null,
• A primary key constraint combines a NOT NULL constraint and a unique constraint in a single declaration, That is, it prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null,
• A foreign key constraint requires values in one table to match values in another table,