geog475.3.23 - e designat es a weak entity set; the key...

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Unformatted text preview: e designat es a weak entity set; the key come par tially from this relationship (shown with a double diamond) Name Host name @ login pr imar y domain Name Name I n2 I n1 Name Secondar y domain hosts K2 K1 K 2 K 1 K 3 colors Has favor ites fans beers addr name name brewer ies Manf. by beers Manf addr manf name Weak entity sets sometimes a key or an entity set comes not completely from its own attributes, but from the keys of one or more entity sets to which it is linked by a {many to one, one to one} relationship This is called a weak entity set . Login name = user name + host name (+ ip add of the host) asdf@adsf.geog.tamu.edu- The password table records the username, not the host (allows someone to login from more than one machine) Consider an IP add consisting of a primary domain, sub-domain and host- Key for primary domain is its name- Key for secondary domain is its name + primary domain name- Key for host = host name + secondary domain name + primary domain name (this info is necessary to uniquely identify a host)...
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geog475.3.23 - e designat es a weak entity set; the key...

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