Relational model includes several types of constraints whose purpose is to maintain the accuracy and integrity of the data in the database. The major types of integrity constraints are:
1. Domain Constraints
All the values that appear in a column of a relation must be taken from the same domain. A domain usually consists of the following components.
- Size or length
- Allowable values or Allowable range (if applicable)
2. Entity Integrity
The Entity Integrity rule is so designed to assure that every relation has a primary key and that the data values for the primary key are all valid. Entity integrity guarantees that every primary key attribute is non null. No attribute participating in the primary key of a base relation is allowed to contain nulls. Primary key performs unique identification function in a relational model. Thus a null primary key performs the unique identification function in a relation would be like saying that there are some entity that had no known identity. An entity that cannot be identified is a contradiction in terms, hence the name entity integrity.
3. Referential Integrity
In the relational model the association between the tables is defined using foreign keys. The association between the SHIPMENT and ELEMENT tables is defined by including the Symbol attribute as a foreign key in the SHIPMENT table. This implies that before we insert a row in the SHIPMENT table, the element for that order must already exist in the ELEMENT table.
A referential integrity constraint is a rule that maintains consistency among the rows of two tables or relations. The rule states that if there is a foreign key in one relation, either each of the foreign key value must match a primary key value in the other table or else the foreign key value must be null.
4. Operational Constraints
These are the constraints enforced in the database by the business rules or real world limitations. For example if the retirement age of the employees in a organization is 60, then the age column of the employee table can have a constraint “Age should be less than or equal to 60”. These kinds of constraints enforced by the business and the environment are called operational constraints.