Constraints

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Constraints

An E-R enterprise schema may define certain constraints to which the contents of a database must conform. In this section, we examine mapping cardinalities and participation constraints, which are two of the most important types of constraints.

Mapping Cardinalities

Mapping cardinalities, or cardinality ratios, express the number of entities to which another entity can be associated via a relationship set.

Mapping cardinalities are most useful in describing binary relationship sets, al- though they can contribute to the description of relationship sets that involve more than two entity sets. In this section, we shall concentrate on only binary relationship sets.

For a binary relationship set R between entity sets A and B, the mapping cardinality must be one of the following:

• One to one. An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A. (See Figure 2.4a.)

• One to many. An entity in A is associated with any number (zero or more) of entities in B. An entity in B, however, can be associated with at most one entity in A. (See Figure 2.4b.)

• Many to one. An entity in A is associated with at most one entity in B. An entity in B, however, can be associated with any number (zero or more) of entities in A. (See Figure 2.5a.)

• Many to many. An entity in A is associated with any number (zero or more) of entities in B, and an entity in B is associated with any number (zero or more) of entities in A. (See Figure 2.5b.)

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The appropriate mapping cardinality for a particular relationship set obviously de- pends on the real-world situation that the relationship set is modeling.

As an illustration, consider the borrower relationship set. If, in a particular bank, a loan can belong to only one customer, and a customer can have several loans, then the relationship set from customer to loan is one to many. If a loan can belong to several customers (as can loans taken jointly by several business partners), the relationship set is many to many. Figure 2.3 depicts this type of relationship.

Participation Constraints

The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R. If only some entities in E participate in relationships in R, the participation of entity set E in relationship R is said to be partial. For example, we expect every loan entity to be related to at least one customer through the borrower relationship. Therefore the participation of loan in

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the relationship set borrower is total. In contrast, an individual can be a bank customer whether or not she has a loan with the bank. Hence, it is possible that only some of the customer entities are related to the loan entity set through the borrower relationship, and the participation of customer in the borrower relationship set is therefore partial.

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