CODEWITHSUNDEEP
erd
Jayuji
Jayuji

Reputation: 15

Differences between has and contains relationship in ERD?

ERD from Wikipedia:

ERD from Wikipedia

I'm confused to distinguish "has" and "contains" relationship in ERD, what's the difference between them?

Upvotes: 1

Views: 4665

Answers (1)

reaanb
reaanb

Reputation: 10064

With regards to the linked diagram from Wikipedia, both Has and Contains are one-to-many relationships (1..n) with total participation of Character in the relationship (double association line). Total participation means every Character must be related to an Account and a Region.

However, the Has relationship is marked as an identifying relationship (double-bordered diamond) and Character is marked as a weak entity set (double-bordered rectangle). This means that the identifying attributes of Character include the identifying attributes of Account. Character has a weak key CharName, which must be combined with AcctName to fully identify a Character. In other words, the primary key of the Character table will be composite: AcctName, CharName.

Note that these comments apply to the example ERD, not to ERD in general. Has and Contains aren't types of relationships - in general, a relationship can be called anything and have any combination of the features available in ER theory.

Also, see my answer to Is optionality (mandatory, optional) and participation (total, partial) are same?

Upvotes: 4

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