46 Weak Entity Sets Occasionally, entities of an entity set need “help” to identify them uniquely. Entity set E is said to be weak if in order to identify entities of E uniquely, we need to follow one or more many- one relationships from E and include the key of the related entities...
36 Keys uA key is a set of attributes for one entity set such that no two entities in this set agree on all the attributes of the key. wIt is allowed for two entities to agree on some, but not all, of the key attributes. uWe must designate a key for every entity set. 37 Keys ...