162 Partiiidatabase and Database ApplicationDesig n

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162 P a r t I I I D a t a b a s e and Database Application D e s i g n

For an ID‑dependent weak entity, it is necessary to add the key of the parent

entity to the weak entity's relation, and this added attribute becomes part of the

weak entity's key. Thus in Figure 64(c), we have added InvoiceNumber, the key of

INVOICE, to the attributes in LINE‑ITEM. The key of LINE‑ITEM is the compos­-

ite (InvoiceNumber, LineNumber). With relations that represent ID‑dependent

weak entities, the key always is composite. Now consider the representation of E‑R

model relationships.

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