Problems

Entity relationship modelling is simple enough in theory, but many situations will arise that need careful thought. There are also a couple of common error situations that you need to be aware of. They arise quite often because they are not immediately obvious.

Fan traps

A fan trap occurs when a model represents a relationship between entity types, but the pathway between certain entity occurrences is ambiguous. It occurs when 1:* relationships fan out from a single entity.

A single site contains many departments and employs many staff. However, which staff work in a particular department?

The fan trap is resolved by restructuring the original ER model to represent the correct association.

Chasm traps

A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences.

It occurs where there is a relationship with partial participation, which forms part of the pathway between entities that are related.

• A single branch is allocated many staff who oversee the management of properties for rent. Not all staff oversee property and not all property is managed by a member of staff.
• What properties are available at a branch?
• The partial participation of STAFF and PROPERTY in the oversees relation means that some properties cannot be associated with a branch office through a member of staff.
• We need to add the missing relationship which is called has between the BRANCH and the PROPERTY entities.
• NB. You need to be careful when you remove relationships which you consider to be redundant.