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These lecture notes from cs 180 cover topics related to weak entity sets, entity-relationship design, and the relational model. The concept of weak entity sets, their representation, and when they are necessary. It also includes examples of weak entity sets and their relationships with other entities. Additionally, it discusses design principles for creating a database schema and the difference between entity sets and attributes.
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Winter 2002
Arthur Keller – CS 180
2–
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Weak Entity Sets, Entity-Relationship Design. ◆
Read Sections 2.3-2.4.
-^
Relational Model, Functional Dependencies. ◆
Read Sections 3.1-3.5.
-^
Normal Forms, Multivalued Dependencies. ◆
Read Sections 3.6-3.7. Assignment 1 due.
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Relational Algebra. ◆
Read Chapter 5. Project Part 1 due.
Winter 2002
Arthur Keller – CS 180
2–
Sometimes an E.S.
’s key comes not (completely) from its own
attributes, but from the keys of one or more E.S.’s to which
is
linked by a
supporting
many-one relationship.
Called a
weak
Represented by putting double rectangle around
and a double
diamond around each supporting relationship.
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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 toextract key attributes by following the supporting relationship.
Winter 2002
Arthur Keller – CS 180
2–
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-^
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2ndaryDomains
PrimaryDomains
@In
name
name
Hosts
@In
name
Winter 2002
Arthur Keller – CS 180
2–
All “Connecting”
Entity SetsAre Weak
a price, we can omit
price
from the key, and
remove the double diamond from
ThePrice
.
price
is attribute of
BBP
.
Bars
Beers
The-Bar
Beers
The-Beer
The-Price
The-Bar
The-Beer
The-Price
name
manf
name
addr
price
Winter 2002
Arthur Keller – CS 180
2–
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Buyer
Product
Qty
Buyer
Product
Shipment
Name
Name
Ordered
OBOB
OPOP
Qty Ordered
Ordered
OBOB
OPOP
Part of
Qty Shipped
Part-of ismany-many andnot a weakrelationship!
Winter 2002
Arthur Keller – CS 180
2–
Beers
Manfs
ManfBy
name
addr
name
Winter 2002
Arthur Keller – CS 180
2–
Winter 2002
Arthur Keller – CS 180
2–
-^
Beers
Manfs
ManfBy
name
name
Beers name
manf
Winter 2002
Arthur Keller – CS 180
2–
The following design illustrates both points:•^
Manfs
deserves to be an E.S. because we record
addr
, a
nonkey attribute.
-^
Beers
deserves to be an E.S. because it is at the “many” end.
◆^
If not, we would have to make “set of beers” an attribute of
Manfs
something we avoid doing, although some may tell you it is OK inE/R model.
Beers
Manfs
ManfBy
name
addr
name
Winter 2002
Arthur Keller – CS 180
2–
There is a tendency to feel that no E.S. has its entitiesuniquely determined without following somerelationships.
-^
However, in practice, we almost always create uniqueID's to compensate: social-security numbers, VIN's, etc.
-^
The only times weak E.S.'s seem necessary are when:a)^
We can't easily create such ID's; e.g., no one is going to accept a“species ID” as part of the standard nomenclature (species is aweak E.S. supported by membership in a genus). b)
There is no global authority to create them,
e.g
., crews and
studios.
Winter 2002
Arthur Keller – CS 180
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