slides2 - Schedule Today Jan 8(T x x Weak Entity Sets...

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Winter 2002 Arthur Keller – CS 180 2–1 Schedule Today: Jan. 8 (T) Weak Entity Sets, Entity-Relationship Design. Read Sections 2.3-2.4. Jan. 10 (TH) Relational Model, Functional Dependencies. Read Sections 3.1-3.5. Jan. 15 (T) Normal Forms, Multivalued Dependencies. Read Sections 3.6-3.7. Assignment 1 due. Jan. 17 (TH) Relational Algebra. Read Chapter 5. Project Part 1 due.
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Winter 2002 Arthur Keller – CS 180 2–2 Weak Entity Sets Sometimes an E.S. E ’s key comes not (completely) from its own attributes, but from the keys of one or more E.S.’s to which E is linked by a supporting many-one relationship. Called a weak E.S. Represented by putting double rectangle around E and a double diamond around each supporting relationship. Many-one-ness of supporting relationship (includes 1-1) essential. With many-many, we wouldn't know which entity provided the key value. “Exactly one” also essential, or else we might not be able to extract key attributes by following the supporting relationship.
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Winter 2002 Arthur Keller – CS 180 2–3 Example: Logins (Email Addresses) Login name = user name + host name, e.g., [email protected] . A “login” entity corresponds to a user name on a particular host, but the passwd table doesn’t record the host, just the user name, e.g., ark . Key for a login = the user name at the host (which is unique for that host only) + the IP address of the host (which is unique globally). Design issue: Under what circumstances could we simply make login-name and host-name be attributes of logins, and dispense with the weak E.S.?
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