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Traps in Entity Relationship Diagrams: Fan Trap and Chasm Trap

by Pritam on 28 Oct 2017

This is an example of fan trap and chasm trap and how to resolve these scenarios. The following brief explanation may help you in understanding the concept which is usually found to be dubious across the web. This explanation is made in parity with the concept explained in Wikipedia (https://en.wikipedia.org/wiki/Entity–relationship_model#Model_usability_issues). You can also refer to https://db.grussell.org/section005.html to understand in detail.

Fan Trap: A (master) table linking to multiple tables in a one-to-many relationship.

An example in different notation, so that you can understand it in your preferred notation.

This becomes problematic when you want to run some query like to get the department for a particular employee. The solution is to adjust your ER model like the following (shown in only the crow's foot notation with the fan trap above and the remodelled version below which resolves the trap).

Chasm Trap: A model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences. Following is the example in different notations.

This becomes problematic if you want to do some query like to get the information about all the computers in a building where you may have been putting computers outside the rooms as well, say, in the reception counter, or in the toilet. Therefore, chasm trap is a case of failure to capture all the relationships that exist in the real world in the model. And hence, to resolve this a new relationship can be introduced as below.

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