Cardinality

Cardinality places a constraint on the number of times an entity can appear in a relationship. The cardinality of a 1:1 relationship is always one. But the cardinality of a 1:n relationship is open; n can be any number. If you must place an upper limit on n, you specify a cardinality for the relationship. For instance, in a store sale example, you can limit the number of sale items that a customer can purchase at one time. You usually use your application program or stored procedure language (SPL) to place cardinality constraints.