Constraint

You can place constraints to limit the type of data that can go into a table. Such constraints can be specified when the table when the table is first created via the CREATE TABLEstatement, or after the table is already created via the ALTER TABLE statement.
Common types of constraints include the following:

• NOT NULL Constraint: Ensures that a column cannot have NULL value.
• DEFAULT Constraint: Provides a default value for a column when none is specified.
• UNIQUE Constraint: Ensures that all values in a column are different.
• CHECK Constraint: Makes sure that all values in a column satisfy certain criteria.
• Primary Key Constraint: Used to uniquely identify a row in the table.
• Foreign Key Constraint: Used to ensure referential integrity of the data.

Each constraint is discussed in the following sections.