Cardinality

The cardinality of a relation is how many instances there can be of each entity that connect to the relation. For example, Then for each subject there are L students, and M staff teach N subjects, as each member of staff may teach several courses. In general, there are 3 different cardinalities:

• One to One, where each entity is related to at most one of the other types of entity. E.g. Each husband has at most one current wife, and each wife has at most one current husband.

• One to Many, where each instance of an entity on one side is related to numerous instances of the other type of entity. E.g. a footballer is related to only one club, but that club may have many footballers.

• Many to Many, e.g. A teacher may teach several subjects at different times, and a subject may have several teachers for different days.

Constraints

For each attribute there may be several constraints placed upon what values it can take:

• Domain of the value

• Format of values (dd/mm/yyyy)

• Range of values (e.g. 1 to 10)

• allow nulls ? Does the attribute have to contain a value for each instance of the entity?

Example of Optionality and Cardinality
 

Attributes

Each entity type can always be described in terms of attributes, and these attributes will apply to all occurrences of that given entity type. In the camera shop example, all occurrences of the entity 'supplier' could be described by an identifiable set of attributes, including:

The Supplier Name, the Supplier Address, Telephone Number, etcetera.

A given attribute belonging to a given entity occurrence can only have one value. Therefore, if a supplier could have more than one address or telephone number then this should be determined before defining the attributes of that entity type.

In this example the defined entity may require two or three address and/or telephone number attributes. It is the maximum practical instances of a given attribute that should be catered for in the entity type definition.