CODEWITHSUNDEEP
databasedatabase-designrelational-database
Bastian
Bastian

Reputation: 1593

ERM: Cardinality in ternary relationships

How can I read and/or specify cardinality (min/max) in ternary relationships of Entity-Relationship-Models (Chen-Notation)?

For example: A head visits several qualification with his associated employees. Now, I want to know how to specify the cardinalitys in Min-Max-Notation.

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Here is another example with given cardinalitys and I want to know how to read it.

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Upvotes: 6

Views: 8659

Answers (1)

saper0
saper0

Reputation: 374

The ternary relationship can be read as follows:

For a given instance of E1 and a given instance of E2, there can only be one instance of E3 associated to both (via R).

For a given instance of E1 and E3, there can only be one instance of E2 associated to both (via R).

For a given instance of E2 and E3, there can be multiple instances of E1 associated to both (via R).

To specify the cardinalities in min-max notation, you have to ask: if I have an instance of "Head", how many instances of Employee and Qualification together can be associated to one "Head" (via HEQ). If your answer would be min 1 and max N you would write (1, N) or (1, *) above the line connecting "Head" and HEQ. Now you proceed to the next entity asking the equivalent question for this entity.

Upvotes: 12

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