Integrity Constraints

Integrity Constraints are conditions that must be satisfied by the database at all times. Some commonly used Integrity Constraints are:

• not null: Certain columns in a table may be prohibited from having null values.
• primary key: is a set of column names which uniquely identify each row.
  Example: {CID} is the primary key for customers table.
  The primary key constraint indicates that the table cannot contain two different rows with the same values under primary key columns.

• candidate key: A relation (table) may contain more than one key.
  In this case, one of the keys is selected as the primary key and the remaining keys are called candidate keys.
  The candidate key constraint indicates that the table cannot contain two different rows with the same values under candidate key columns.
• foreign keys (referential integrity): Many a times, it is necessary to include column names from one table in another table.
  Example: CID, AID and PID are included in the orders table.
  CID is a primary key of the customers table and its inclusion in the orders table makes it a foreign key in the orders table.
  This constraint forces every CID value in the orders table to be also present in the customers table.