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.
Share with your friends: | 
