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It does not matter which way around it is done but you should not have a foreign key in each entity.The primary key of one entity type comes the foreign key in the other.If the two entity types are kept separate then the association between them must be represented by a foreign key. efficiency considerations when fast responses are required or different patterns of updating occur to the two different entity types.the entities participate in very different relationships with other entities.the two entity types represent different entities in the `real world'.There are a few reason why you might not combine a 1:1 mandatory relationship. The primary key of the new combined entity is usually the same as that of the original more important entity type.If there are any attributes in common, the duplicates are removed.The key of the subsumed entity type becomes a normal attribute.The result of this amalgamation is that all the attributes of the `swallowed up' entity become attributes of the more important entity.The choice of which entity type subsumes the other depends on which is the most important entity type (more attributes, better key, semantic nature of them).

If the relationship is mandatory at both ends it is often possible to subsume one entity type into the other. mandatory at one end and optional at the other.There are three possibilities the relationship can be: Mapping 1:1 relationshipsīefore tackling a 1:1 relationship, we need to know its optionality. This is the ideal time to check the model, as it is really the last chance to make changes to the ER model without causing major complications. Foreign keys are usually shown in italics or with a wiggly underline.īefore we start the actual mapping process we need to be certain that we have simplified the ER model as much as possible.It will generally have a foreign key from each table that it is related to.A relation can have several foreign keys.

They allow the relations to be linked together.

They are added to relations as we go through the mapping process.

Roughly, each foreign key represents a relationship between two entity types.

When referring to relations or tables, cardinality is considered to the the number of rows in the relation or table, and arity is the number of columns in a table or attributes in a relation.Ī foreign key is an attribute (or group of attributes) that is the primary key to another relation. Student( matric no, name, address, date_of_birth)

If matric_no was the primary key, and there were no foreign keys, then the table above could be represented as: Tablename( primary key, attribute 1, attribute 2. Relations can be represented textually as:

The individual entities become the rows.

It is two-dimensional and has rows and columns.Įach entity type in the ER model is mapped into a relation.

differences between mapping optional and mandatory relationships.Ī relation is a table that holds the data we are interested in.

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Lecture Slides available: PDF PowerPoint Mapping ER Models into Relations Contents Mapping ER Models into Relations Previous Page