The Entity Relationship Diagram

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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:

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.

Page created and maintained by Osmar R. Zaï ane
Last Update: Sun Sep 10 17:02:35 PDT 1995