Web-DSS-Chapter-03
Web-DSS-Chapter-03
Web-DSS-Chapter-03
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SECTION 3110 ■ Attributes of Relationships 51<br />
Date<br />
STUDENT<br />
Uses<br />
EQUIPMENT<br />
PROJECT<br />
(c)<br />
STUDENT<br />
r1<br />
r2<br />
r3<br />
r4<br />
r5<br />
PROJECT<br />
p1<br />
p2<br />
p3<br />
Uses<br />
r1<br />
r2<br />
r3<br />
r4<br />
EQUIPMENT<br />
e1<br />
e2<br />
e3<br />
e4<br />
e5<br />
(d)<br />
Figure 3.16 (continued)<br />
3.11 Attributes of Relationships<br />
We have already discussed examples of E-R diagrams that reveal attributes stemming off relationships.<br />
Recall Figure 3.15(c), for example. In that E-R diagram, the attribute DateofOrder collects<br />
data for the relationship CUSTOMER places ORDER. Attributes on relationships are like<br />
attributes on entity types we have seen so far. An attribute on a relationship stores information<br />
related to the relationship. In Figure 3.17, the attribute Quantity stores the number of components<br />
that make up an entity type ITEM. Note that the attribute Quantity stems from the relationship<br />
and not from the entity.
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