CS 430
Database Theory
Winter 2005
Lecture 15: How to Convert an ER
Model to Relations
1
Issue
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We do Conceptual Modeling using ER
Models
Our databases are relational
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Look a little like ER models but not exactly
How to we convert an ER model to relations
for an RDBMS
Note: Typically the resulting relations are 3NF
2
ER Model

ER Model has:
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Entities, Relationships, Attributes
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To convert to Relation Tables:
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Relationships may have attributes
May have multi-valued and composite attributes
Need Keys for Entities
Relationships need cardinalities, e.g. 1:1, 1:n, m:n
Example ER Model:
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Figure 7.1 from Text Book
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Steps
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Convert Independent (Strong, Regular)
Entities
Convert Dependent (Weak) Entities
Convert Binary Relations
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1:1
1:n
m:n
Convert N-Ary Relationships (N>2)
Convert Multi-Valued Attributes
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Convert Independent Entities
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Create a new Relation for the Entity
Make all simple (non-multi-valued, noncomposite) attributes of the Entity into
attributes of the Relation
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For single-valued composite attributes, use the
underlying simple attributes
Choose some Key attribute(s) to be the
Primary Key
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Result So Far
EMPLOYEE
SSN
Fname
Minit
LName
Sex
Address
BDate
Salary
DEPARTMENT
DName
DNumber
PROJECT
PName
PNumber
PLocation
6
Convert Dependent Entities


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Create a new Relation for the Entity
Make all simple attributes into attributes of
the relation (same as for Independent
Entities)
Add the primary key attribute(s) of the owning
entity as a foreign key for the weak entity


Convert entities in order: If one weak entity owns
another, convert the owner first
Add ON UPDATE and ON DELETE

The usual approach is CASCADE
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Result So Far
EMPLOYEE
SSN
Fname
Minit
LName
Sex
Address
BDate
Salary
DEPARTMENT
DName
DNumber
PROJECT
PName
PNumber
PLocation
DEPENDENT
ESSN
Dependent_Name
Sex
BDate
Relationship
8
Map 1:1 Relations
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Foreign Key approach:
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Merged Relationship
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Include the primary key of one relation as a
foreign key in the other
Include attributes of the relationship with the
foreign key
Merge the two relations
Cross Reference
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Add a new table with the keys from both entities
(see m:n for example of this)
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Result So Far
EMPLOYEE
SSN
Fname
Minit
LName
Sex
Address
BDate
Salary
DEPARTMENT
DName
DNumber
MgrSSN
MgrStartDate
PROJECT
PName
PNumber
PLocation
DEPENDENT
ESSN
Dependent_Name
Sex
BDate
Relationship
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Map 1:n Relations

Let R be the “1 side”, S the “n side”
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
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Include R’s key as a foreign key in S
Include any attributes of the relation together with
the foreign key in S
Or use the Cross Reference table approach
(m:n approach)
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Result So Far
EMPLOYEE
SSN
Fname
Minit
LName
Sex
Address
BDate
Salary
SuperSSN
DNo
DEPARTMENT
DName
DNumber
MgrSSN
MgrStartDate
PROJECT
PName
PNumber
PLocation
DNum
DEPENDENT
ESSN
Dependent_Name
Sex
BDate
Relationship
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Map m:n Relations
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Create a new Relation to represent the Relationship
Include the keys of the related entities as foreign
keys in the relation
Make the key of the relation the combined keys of
the participating entities
Include any simple attributes of the relationship as
attributes of the relation
Include CASCADE for UPDATE and DELETE
(typical)
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Result So Far
EMPLOYEE
SSN
Fname
Minit
LName
Sex
Address
BDate
Salary
SuperSSN
DNo
DEPARTMENT
DName
DNumber
MgrSSN
MgrStartDate
PROJECT
PName
PNumber
PLocation
DNum
WORKS_ON
ESSN
PNo
Hours
DEPENDENT
ESSN
Dependent_Name
Sex
BDate
Relationship
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Map N-Ary Relationships

Treat the same as mapping m:n relationships

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Create a new relation
Make the keys of all the parents, foreign keys of
the relation
Make the key the combined foreign keys
Copy any simple attributes of the relationship as
attributes of the relation
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Map Multi-Valued Attributes
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Create a new Relation to represent the Attribute
Include the key of the parent Relation as a foreign
key for the new relation
Include the Multi-Valued Attribute as an attribute of
the relation
Make the primary key the combination of the parent
key and the attribute
CASCADE on UPDATE and DELETE
Note: If you have a composite multi-valued attribute,
check to make sure you don’t have a weak entity

Impacts the set of attributes that make up the primary key
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Final Result
EMPLOYEE
SSN
Fname
Minit
LName
Sex
Address
BDate
Salary
SuperSSN
DNo
DEPARTMENT
DName
DNumber
MgrSSN
MgrStartDate
DEPT_LOCATIONS
DNumber
DLocation
PROJECT
PName
PNumber
PLocation
DNum
WORKS_ON
ESSN
PNo
Hours
DEPENDENT
ESSN
Dependent_Name
Sex
BDate
Relationship
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