Page 248 - DCAP405_SOFTWARE_ENGINEERING

Unit 14: Data Modeling

is a multi-valued attribute – i.e., one instance of a branch manager will have a single value for Notes
BranchManagerId but can have more than one phone number.
An attribute can be Derivable or Non-Derivable. Derivable attribute can be obtained by subjecting
other attributes to computation. For instance, Age is a derived attribute for an AccountHolder
entity because its value can be obtained by computing the difference between the current date
and the DateOfBirth attribute.

Notes Since the value of the derived attribute can be computed from other attributes its
value need not be stored in the database.
Self Assessment

Fill in the blanks:
10. On the basis of whether an attribute must take a value for an instance of a given entity or
not, attributes can be either…………………….

11. On the basis of the ………………………. of values taken attributes can be classified either
as Single-valued or Multi-valued.
12. ………………………. attribute can be obtained by subjecting other attributes to computation.

14.4 Relationships and Relationship Set

Entities do not exist in isolation form each other. They are more often than not related somehow
with each other. A relationship is an association between entities. Let there be n number of
entities given by: E , E , E ,…., E n
3
1
2
A relationship is defined as a collection of attributes( r , r ,.... r ) r , such that r ∈ E and
3
2
1
n
r ∈ E and so on. A relationship set is a set of relationships of the same type. 1 1
2 2
Example: Consider the two entity sets – AccountHolders and Account. These two entities
are related to each other because an Account is owned by an Account holder. We can give a name
to this relationship – say, Owns. The relationship would read as:
AccountHolder(Name=”Eknath”, Address=”A-45, Tagore Garden”, DateOfBirth=”13/11/
1966", DateOfOpeningAccount=”12/10/2005") Owns Account (AccountNumber = “1345”,
OpeningBalance=50000)

In fact another relationship may also exist in the opposite manner. In our case the
relationship – Belongs To - can be thought of a relationship from Account entity to
AccountHolder entity as:
Account (AccountNumber = “1345”, OpeningBalance=50000) Belongs To
AccountHolder(Name=”Eknath”, Address=”A-45, Tagore Garden”, DateOfBirth=”13/11/
1966", DateOfOpeningAccount=”12/10/2005")
Thus, the two relationships “Owns” and “Belongs To” are inversely related to each other.
A relationship that exists between two entities is called binary relationship. But a relationship
may exist between more than two entities as well. A relationship between three entity sets is
called ternary relationship, and so on. The number of sets forming a relationship is known as
degree of relationship. Therefore, a binary relationship is of degree 2 while ternary is that of
3 and a quinary of degree 5.

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