7.1. Constraints
Constraints are a way to validate the data in a column or columns of a table. The Oracle database has five distinct types of constraints that can be defined on a column or columns in a table: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, and FOREIGN KEY. Only the FOREIGN KEY constraint, as its name implies, does its validation in reference to another table within the database.
constraint
A condition defined against a column or columns on a table in the database to enforce business rules or relationships between tables in the database.
NOTE
The end-user application frequently validates the data entered into the database, even before an INSERT or UPDATE operation occurs, and this might be the best way to implement complex business rules. ...
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