What is meant by the cardinality of a relationship?
What will be an ideal response?
In the E-R model, there are two types of cardinality—the maximum cardinality and the minimum cardinality. The maximum cardinality is the maximum number of entity instances that can participate in a relationship instance. There are three types of maximum cardinality: one-to-one (1:1 ), one-to-many (1:N) and many-to-many (N:M). The minimum cardinality is the minimum number of entity instances that must participate in a relationship instance. This number is generally zero (0 ) or one (1 ). If the number is zero, then participation by that entity instance is optional (O)—it does not have to be in a relationship with an instance of the other entity. If the number is one, then participation by that entity instance is mandatory (M)—it must participate in a relationship with an instance of the other entity. In a binary relationship there are four (4 ) possible sets of minimum cardinalities—(O-O), (O-M), (M-O) and (M-M).
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