Decomposition

DECOMPOSITION

(Handling Many to Many
relationships)

1

Aims:

◦ To demonstrate how to decompose
many:many (M:N) relationships

◦ To explain why M:N relationships need
to be decomposed.

2

 A M:N relationship between 2 entity types must
be decomposed into two 1:M relationships.

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M M
MODULE
STUDENT chooses

Becomes

1M M 1

STUDENT makes CHOICE for MODULE

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M M
A rB

Becomes 1 M M1
A B

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 Naming the new entity type and the new
relationships is sometimes not easy

 Often the old relationship name will help
you name the new entity. In the example
“chooses” became “choice”

 If all else fails, concatenate (join) the
names of the 2 original entity types (e.g.
StudentModule).

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 Decompose this M:M relationship to form two 1:M
relationships:

M

DOCTOR treat PATIENT

 Assign the new entity and relationship types
suitable names.

7

MM

DOCTOR do TREATMENT for PATIENT

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 Entity must have an identifier / keys.
 The new entity type created by decomposition

needs an identifier

9

Back to the first example
Look at the original M:M relationship:

M M

STUDENT chooses MODULE

Student (student#, name, . . .)

Module (module#, description, . . .)

How do we know which students are taking
which modules?
We don’t.

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M M

ACTOR appears FILM
in

Actor (actor#, name, . . .)
Film (film#, title, . . .)

 Decompose this M:M relationship
 Assign the new entity type an appropriate name

and think of some additional attributes for it
 Assign the new entity type a suitable identifier /

key.

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ACTOR make MM for FILM

APPEARANCE

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