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.

  
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.