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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.
Here is another example with given cardinalitys and I want to know how to read it.
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Before you go through this article, make sure that you have gone through the previous article on Introduction to ER Diagrams. Cardinality constraint defines the maximum number of relationship instances in which an entity can participate. 1. Many-to-Many Cardinality- One student can enroll in any number (zero or more) of courses.
A relationship associating instances of three entities, e.g., sale is termed a ternary relationship (degree 3). Generally a relationship associating instances of N entities is termed an N-ary relationship (degree N). The original ER model supports N-ary relationships.
This is the cardinality to assign to the ternary relationship on the edge that connects the ternary relationship with the entity C. This phrase has to be reformed for all possible combinations (not permutations, since the order of pairing doesn't matter).
You can set the cardinality for the relationships in your model. Relationship cardinality represents the fact that each parent entity or table within a relationship is connected to a particular number of instances of the child entity or table.
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.
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