Constraints are rules that you can add to your table and column definitions.
These rules govern the data values that can be entered into a table or column.
This module discusses how to

1. Identify the four basic types of constraints
2. Create PRIMARY KEY constraints on database tables
3. Create unique constraints on database tables
4. Create foreign key constraints on database tables
5. Identify how CHECK constraints work
6. Use the EXCEPTIONS INTO clause of the CONSTRAINT definition
7. Identify how constraints are implemented
8. Find information about constraints in the data dictionary

Adding constraints to the tables and columns in your database helps guarantee the quality of the data in the database. The lower the quality of the data, the lower the quality of decisions that are made on the data. Since the primary purpose of data collection is to enable intelligent decisionmaking, the proper use of constraints provides much of the power of your Oracle database.