Entity Relationship Modeling

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Entity Relationship Modeling

• BUS112 Professor Kwan

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Entity Relationship Diagram
An Entity Relationship Diagram (ERD) is used to
represent the Entity Relationship Model of a
Logical Schema (Logical View) of a Database. It
describes in a very macro manner the Entities and
Relationships involved in a system. The ERD has
been adopted as the standard for documentation
and communication in industry (but variations
abound).
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Entity
Any distinguishing Object that is to be (or
being) represented. An Object about which we
want to record data. Usually has a Noun as its
name. For example, an Employee. Entity Class - A
Group of Entities with the same Attributes and
are related to each other. For example, Employees.
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Attribute
Entity
A Characteristic of an Entity. Usually has an
Adjective as its name. Its value must come from
the Attributes Domain. Domain - described by
Type, Size and the set of Possible and Valid
values.
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Relationship
Entity
Entity
Connections or mappings superimposed on the
Entities. Could be within or among Entity
Classes. Usually has a Verb as its name. Binary
Relationships - Basic building blocks.
1 1 One to One 1 M One to
Many (Parent/Child) M N Many to Many
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Relationship
1 1 One to One
Entity B
Entity A
Entity B
1 M One to Many
Entity A
Entity B
M N Many to Many
Entity A
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The Cardinality of a Relationship
Two Notations
(0,1)
Min 0 Max 1 Zero or One (Optional)
Min 1 Max 1 One and Only One (Mandatory)
(1,1)
Min 0 Max Many Zero or Many (Optional)
(0,N)
(1,N)
Min 1 Max Many One or Many (Mandatory)
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Some Common Relationship Structures
A Tree or Hierarchy Each Entity has only one
Parent
A
1 M
A
B
B
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Some Common Relationship Structures
A Simple Network Entities can have more than one
Parent
A
B
A
B
1 M
1 M
1 M
C
D
C
D
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Some Common Relationship Structures
A Complex Network Entities can have more than one
Parent
A
A
M N
B
B
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Some Common Relationship Structures
A Unary Relationship (1 M or M N) Entity is
related to Itself
A
A
M N
Sometimes called a Recursive or the
Bill-of-Material (BOM) Relationship.
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Bank Database Schema Example
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Inventory Database Schema Example