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14.1 Introduction
Database systems typically have only a very limited understanding of what the data in the database means.
Semantic modeling ideas can be useful as an aid in the process of database design, even in the absence of direct DBMS support for those ideas.
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14.2 The Overall Approach
Four steps:
1. To identify a set of semantic concepts that seem to be useful in talking informally about the real world.
e.g., entities, entity types, instances
properties, identity, relationships
2. To devise a set of corresponding symbolic objects that can
be used to represent the foregoing semantic concepts.
e.g., E-relations, P-relations
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4. 14.2 The Overall Approach (Cont.)
3. To devise a set of formal, general integrity rules to go along with those formal objects.
e.g., property integrity
4. To develop a set of formal operators for manipulating those formal objects.
e.g., Property operator
Semantic concepts (See Fig (next slide))
The very same object in the real world might legitimately be regarded as an entity by some people, as a property by others, as a relationship by still others.
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Fig Semantic concepts
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14.3 The E/R Model
The entity/relationship model introduced by Chen in 1976.
E/R diagrams (See Fig (next slide))
Entities: a thing which can be distinctly identified.
regular or strong entities
weak or dependent entities
Properties
simple or composite
key
single or multi-valued
missing
base or derived
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Fig E/R diagrams
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14.3 The E/R Model (Cont.)
Relationships: an association among entities
participants
the degree of the relationship
total or partial
one-to-one, one-to-many, many-to-many
The many-to-many case is the only one that demands
representation by means of a separate relvar.
Entity Subtypes and Supertypes
subtype vs. supertype
inheritance
a. properties
b. relationships
entity type hierarchy (See Fig. 14.3)
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9. Fig. 14.3 Entity type hierarchy
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14.4 E/R Diagrams
The popularity of the E/R model as an approach to database design can probably be attributed more to the existence of the E/R diagramming technique.
Entities: a rectangle
regular entity type  a rectangle
e.g., Department, Employee, Supplier, Part, Project
weak entity type  the border of the rectangle is doubled.
e.g., Dependent
Properties: an ellipse
derived  dotted or dashed
multi-valued  doubled
composite  further ellipses
key  underlined
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14.4 E/R Diagrams (Cont.)
Relationships: a diamond
between a weak entity type and an entity type:
the diamond border is doubled
one-to-one, many-to-one: the line is labeled “1” or “M”
total  the line is doubled
e.g., many-to-one: Dept_Emp, Emp_Dep, Proj_Manager
many-to-many: Proj_Work, Supp_Part_Proj,
Supp_Part, Part_Structure
Entity Subtypes and Supertypes
“the isa relationship”  a solid line marked with an arrowhead
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12. 14.5 Database Design with the E/R Model (1/5)
An E/R diagram  a relational database definition
Regular entities: a base relvar
e.g., Var Dept Base Relation
{ Dept# …, … }
Primary Key {Dept#};
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13. 14.5 Database Design with the E/R Model (2/5)
Many-to-many relationships: a base relvar
e.g., Var SP Base Relation
{ S# ..., P#..., ... }
Primary Key {S#,P#}
Foreign Key {S#} References S
On Delete Restrict
On Update Cascade
Foreign Key {P#} References P
On Update Cascade;
Two participants  two foreign keys
If that combination is unique, and if the designer has no objection to composite primary keys  two foreign keys as the primary key
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14. 14.5 Database Design with the E/R Model (3/5)
Many-to-one relationships:
To introduce a foreign key in the relvar on the “many” side of the relationship that references the relvar on the “one” side.
e.g., Var Emp Base Relation
{ Emp# ..., Dept#..., ... }
Primary Key {Emp#}
Foreign Key {Dept#} References Dept
On Delete ...
On Update ...;
One-to-one relationships: the same way as many-to-one
relationships
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15. 14.5 Database Design with the E/R Model (4/5)
Weak entities: a many-to-one relationship
The Delete and Update rules for the relationship must be as follows:
On Delete Cascade
On Update Cascade
e.g., Var Dependent Base Relation
{ Emp# ..., … }
Primary Key { Emp#, Dep_name }
Foreign Key {Emp#} References Emp
On Update Cascade;
If the database designer has no objection to composite
primary keys  the combination of the foreign key and the
weak entity “key”
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16. 14.5 Database Design with the E/R Model (5/5)
Properties: An attribute
Entity Supertypes and Subtypes: a base relvar
e.g. Var Pgmr Base Relation
{ Emp# ..., Lang ..., ... }
Primary Key {Emp#}
Foreign Key {Emp#} References Emp
On Delete Cascade
On Update Cascade;
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14.6 A Brief Analysis
The E/R model as a Foundation for the Relational Model?
The relational model is a formal system; the E/R model is not a formal model.
The relational model was originally and explicitly based on some rather E/R-like ideas.
Is the E/R Model a Data Model?
The E/R model is indeed a data model, but one that is
essentially just a thin layer on top of the relational model.
The fundamental E/R data object  the n-ary relation
The E/R operators  the operators of the relational algebra
The integrity  a given relvar represents a certain kind of relationship
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18. 14.6 A Brief Analysis (Cont.)
Entities vs. Relationships
The very same object can quite legitimately be regarded as an entity by some users and a relationship by others.
Example: a marriage
Relationship  “Who was Elizabeth Taylor married to in 1975?”
Entity  “How many marriages have been performed in this church since April?”
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The End.
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