Cardinality in Software

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In every relationship we ve discussed, we ve specified only the general type of relationship one-to-one, one-to-many, and many-to-many In each case, the description of the relationship is a specification of the number of records in a parent entity in relation to the number of records in a child entity To more clearly model the actual relation of the data, you can be more specific when defining these relationships What you are specifying is the cardinality of the relationship With a one-to-one relationship, the cardinality is implied You are clearly stating that for every one record in the parent entity, there might be one record in the child entity It would be more specific to say that there is zero or one record in the child entity for every one record in the parent entity But if you mean to say that there absolutely must be a record in each entity, then the relationship s cardinality would be one record in the child entity for every one record in the parent entity The cardinality of a one-to-one relationship is notated as [1:1] In a one-to-many relationship, notated as [1:M], the cardinality implied is one or more records in the child entity for every one record in the parent entity But if the intent is that there doesn t need to be a record in the child entity, then the alternative definition is zero or more records in
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Elements Used in Logical Data Models
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the child entity for every one record in the parent entity In most relationships, the zero or more to many interpretation is correct, so be sure to specify and document the alternative definition if it s used in your model A many-to-many relationship could be defined as zero or more to zero or more records In this case, the zero or more to zero or more records cardinality is almost always implied, although you could specify that there must be at least one record in each entity In this case, show a many-tomany as [M:M] In some data modeling software, you can specify that there be an explicit cardinality, such as eight records in the child entity for every one record in the parent entity For example, you may want to model managers to direct reports (business lingo for people who report directly to that manager ) The company may state that to be a manager you must have at least four and no more than twenty direct reports In this example, the cardinality would be at least four and no more than twenty to one Be sure to document this type of cardinality if your business requirements dictate it, because most people will assume the cardinality based on the definitions given here
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Using Subtypes and Supertypes
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When you are determining the entities to be used in a data model, occasionally you may discover a single entity that seems to consist of a number of other complete entities When this happens, it can be confusing when you try to determine which attributes belong to which entities and how to relate them The answer to this dilemma is to use a supertype
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Supertypes and Subtypes Defined
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A supertype is an entity that has multiple child entities, known as subtypes, which describe variations of the same type of entity A collection of a supertype with its subtypes is sometimes referred to as a subtype cluster These most commonly occur when you re dealing with categories of specific things, as shown in the simple example in Figure 27 Assume that we re tracking information about broadband products In this example, BroadBand is an entity, with appropriate attributes and a primary key However, we want to divide the types of broadband into their
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