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entity-relationship diagram

Entity-Relationship Diagrams (ERD)

Entity-Relationship Diagrams

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The Entity Relationship Diagram

We can express the overall logical structure of a database graphically with an E-R diagram.

Its components are:

• rectangles representing entity sets.
  
• ellipses representing attributes.
  
• diamonds representing relationship sets.
  
• lines linking attributes to entity sets and entity sets to relationship sets.
  

In the text, lines may be directed (have an arrow on the end) to signify mapping cardinalities for relationship sets. Figures 2.8 to 2.10 show some examples.

Figure 2.7: An E-R diagram

Figure 2.8: One-to-many from customer to account

Figure 2.9: Many-to-one from customer to account

Figure 2.10: One-to-one from customer to account

Go back and review mapping cardinalities. They express the number of entities to which an entity can be associated via a relationship.

The arrow positioning is simple once you get it straight in your mind, so do some examples. Think of the arrow head as pointing to the entity that ``one'' refers to.

Primary Keys for Relationship Sets

The attributes of a relationship set are the attributes that comprise the primary keys of the entity sets involved in the relationship set.

For example:

• S.I.N. is the primary key of customer, and
  
• account-number is the primary key of account.
  
• The attributes of the relationship set custacct are then (account-number, S.I.N.).
  

This is enough information to enable us to relate an account to a person.

If the relationship has descriptive attributes, those are also included in its attribute set. For example, we might add the attribute date to the above relationship set, signifying the date of last access to an account by a particular customer.

Note that this attribute cannot instead be placed in either entity set as it relates to both a customer and an account, and the relationship is many-to-many.

The primary key of a relationship set depends on the mapping cardinality and the presence of descriptive attributes.

With no descriptive attributes:

• many-to-many: all attributes in .
  
• one-to-many: primary key for the ``many'' entity.
  

Descriptive attributes may be added, depending on the mapping cardinality and the semantics involved (see text).

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