Most relational databases have declarative referential integrity. The referential integrity constraint states that the customer ID (CustID) in the Order table must match a valid CustID in the Customer table.

Recall that a typical join is between a PK and FK. To ensure that there are no orphan records, we need to enforce referential integrity. An orphan record is one whose foreign key FK value is not found in the corresponding entity – the entity where the PK is located. It means the reference from a row in one table to another table must be valid.Įxamples of referential integrity constraint in the Customer/Order database of the Company : Referential integrity requires that a foreign key must have a matching primary key or it must be null. This constraint is specified between two tables (parent and child) it maintains the correspondence between rows in these tables. For example, in the EMPLOYEE table, Phone cannot be a primary key since some people may not have a telephone. This is because null values for the primary key mean we cannot identify some rows. Neither the PK nor any part of it can contain null values. To ensure entity integrity, it is required that every table have a primary key. There are several kinds of integrity constraints, described below. Such information is provided in logical statements called integrity constraints. For example, the Employee ID (EID) must be unique or the employee Birthdate is in the range. We need more specific ways to state what data values are or are not allowed and which format is suitable for an attribute. However, there are real-world semantics for data that cannot be specified if used only with domain constraints. Domain Integrityĭomain restricts the values of attributes in the relation and is a constraint of the relational model. Constraints are the rules that force DBMSs to check that data satisfies the semantics.

Constraints are useful because they allow a designer to specify the semantics of data in the database. In fact, the relational model supports the well-defined theory of constraints on attributes or tables. Chapter 9 Integrity Rules and ConstraintsĬonstraints are a very important feature in a relational model.

Importance of constraint theory