Database ER Model Summary

Avoid redundancy

The definition of redundancy expresses the same transaction in different ways.

Redundancy results in space waste and data inconsistency.

 

 

 

The figure on the right shows the manufacter of beer twice, which is the attribute of beers and the object manfs.

 

The problem with this design is that if a manufacter does not have beers, its address will be lost.

 

Indicates that the entity set must meet at least one of the following two conditions.

1. There must be at least one non-key attribute. That is, it is not just a name.

2. One party in the Multi-to-many or multi-to-one relationship.

 

Beers, in the multiple-to-one relationship, manfs has a non-key attribute address.

If manfs only has the name attribute, it does not need to be an entity. It can be used as the Beers attribute.