- Mapping Cardinalities: express the number of entities to which another entity can be associated via a relationship. For binary relationship sets between entity sets A and B, the mapping cardinality must be one of:
- One-to-one: An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A. (Figure 2.3)
- One-to-many: An entity in A is associated with any number in B. An entity in B is associated with at most one entity in A. (Figure 2.4)
- Many-to-one: An entity in A is associated with at most one entity in B. An entity in B is associated with any number in A. (Figure 2.5)
- Many-to-many: Entities in A and B are associated with any number from each other. (Figure 2.6)
- Existence Dependencies: if the existence of entity X depends on the existence of entity Y, then X is said to be existence dependent on Y. (Or we say that Y is the dominant entity and X is the subordinate entity.)For example,
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