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3.3 Entity Types, Entity Sets, Attributes, and Keys
Some entity types have more than one key attribute. For example, each of the
Vehicle_id and Registration attributes of the entity type CAR (Figure 3.7) is a key in its
orvn right. The Registration attribute is an example of a composite key formed from
trvo simple component attributes, State and Number, neither of which is a key on its
orvn. An entity type may also have no key,in which case it is called a weak entity type
see Section 3.5).
Value Sets (Domains) of Attributes. Each simple attribute of an entity type is
associated with a value set (or domain of values), which specifies the set of values
that may be assigned to that attribute for each individual entity. In Figure 3.6, if the
range of ages allowed for employees is between 16 and 70, we can specifr the value
let of the Age attribute of eVpLOYf E to be the set of integer numbers between 16
and 70. Similarly, we can specify the value set for the Name attribute to be the set of
strings ofalphabetic characters separated by blank characters, and so on. Value sets
are not displayed in ER diagrams. Value sets are typically specified using the basic
data types available in most programming languages, such as integer, string,
Boolean, float, enumerated type, subrange, and so on. Additional data types to represent
date, time, and other concepts are also employed.
(a)
(b)
CAR
Registration (Number, State), Vehicle_id, Make, Model, Year, {Color}
cARl
((ABC 123, TEXAS), TK629, Ford Mustang, convertible, 2004 {red, black})
cAR2
((ABC 123, NEW YORK), WP9872, Nissan Maxima, 4-door, 2005, {blue})
cAR3
((VSY 720, TEXAS), fD729, Chrysler LeBaron, 4-door, 2002, {white, blue})
Figure 3.7
The CAR entity type with two
key attributes, Registration and
Vehicle_id, (a) ER diagram
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