SQL constraints concepts

Constraints are used to limit the type of data that can go into a table.

Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).

Here are several SQL constraints:

• NOT NULL
• UNIQUE
• PRIMARY KEY
• FOREIGN KEY - A FOREIGN KEY in one table points to a PRIMARY KEY in another table.
• CHECK
• DEFAULT