Discuss about Primary of a Weak Entity Set
OBTAINING PRIMARY OF A WEAK ENTITY SET :-
The primary key of a weak entity set can be obtained as follows :-
Case 1 àThe weak entity set is dependent on a strong entity set and has a discriminator. The discriminator of a weak entity set is an attribute or a set of weak entities related to one strong entity whose primary key value is given.
For exam. The discriminator of weak entity set DEPARTURE is the attribute DATE. Hence the primary key of the entity set DEPARTURE is {File No, Date }.
Case 2 à The weak entity set is dependent on two distinct strong entity sets. In this case, the primary key of the weak entity set is form by the two primary keys of the two strong entity set.The entity set QUOTATIONS is a weak entity set which is depending on two entity sets SUPPLIERS and ITEMS. The weak entity set has no discriminator. A QUOTATION entity is uniquely identified only when both the supplier who has furnished it and the item for which it has been furnished are known. Thus the primary key of the weak entity set QUOTATION is { S-name, S-Add, I-No }.
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