ER- Relational Mapping (Based on Chapter 7 in Fundamentals of Database Systems by Elmasri and Navathe, Ed. 4)

ER- Relational Mapping

(Based on Chapter 7 in Fundamentals of Database Systems by Elmasri and Navathe, Ed. 4)

Mapping from ER Model to Relational Model

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Content

Relational Database Design Using ER-to-Relational Mapping ER-to-Relational Mapping Algorithm Summary of Mapping for Model

Constructs and Constraints


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ER-to-Relational Mapping Algorithm

STEP 1 Strong Entity TypesSTEP 2 Weak Entity TypesSTEP 3 Binary 1 to 1 Relationship TypesSTEP 4 Binary 1 to N Relationship TypesSTEP 5 Binary M to N Relationship TypesSTEP 6 Multivalued AttributesSTEP 7 n-ary Relationship Types


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STEP 1 Strong Entity Types Mapping

For each regular (strong) entity type E in the ER schema, create a relation R that includes all the simple attributes.

Include only the simple component attributes of a composite attribute.


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STEP 1 Continued

Choose one of key attributes of E as primary key of R.

If the chosen key of E is composite, the set of simple attributes that form it will together form the primary key of R.


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Entity Types Mapping

ETET

ET BET

B

ET AET

BA

B


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Composite Attributes Mapping

ET

BA D EC

E

D

ETA B


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STEP 2 Weak Entity Types Mapping

For each weak entity type W in the ER schema with owner entity type E, create a relation R, and include all simple attributes (simple

component of composite attributes) of W as attributes of R.


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STEP 2 Continued

In addition, include as foreign key attributes of R the primary key attribute(s) of the owner entity type(s);

This takes care of the identifying relationship type of W.


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STEP 2 Continued

The primary key of R is the combination of the primary key(s) of the owner(s) and the partial key of the weak entity type W, if any.


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STEP 2 Continued

It is common to choose the propagate (CASCADE) option for the referential triggered action on the foreign key in the identifying entity relation of the weak entity type, since a weak entity has an existence dependency on its owner entity.

This can be used for both ON UPDATE and ON DELETE.


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Weak entities

ET2 B

R

ET1

A

ET2A

ET1

A

B


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STEP 3 Binary 1:1 Relationship Types Mapping

For each binary 1:1 relationship type R in the ER schema, and S and T, the relations that participate in R Choose one relation, for example S,

and include as foreign key in S the primary

key of T.


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1 to 1 Relationship Mapping

R

1

ET2

ET1

1

ET2

B

ET1

A B

ET1

A

ET2

B A

- or -


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STEP 3 Continued

It is better to choose an entity type with total participating in R as the role of S.

Include all simple attributes ( or simple component of composite attributes) of the 1:1 relationship type R as attributes of S.


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1 to 1 relationship

R

1

ET2

ET1

1

ET1

A

ET2

B A


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STEP 4 Binary 1:N Relationship Types Mapping

For each regular binary 1:N relationship type R, identify the relations S that represents the participating entity type at the N-side of the relationship type.


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STEP 4 Continued

Include as foreign key in S the primary key of T that represents the other entity type (the 1-side) participating in R;

this is because each entity instance on the N-side is related to at most one entity instance on the 1-side of the relationship type.


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STEP 4 Continued

Include any simple attributes (or simple component of composite attributes) of the 1:N relationship type as attributes of S.


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1 to n Relationship Mapping

R

1

ET2

ET1

n

ET1

A

ET2

B A

ET2

B

ET1

A

NO


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STEP 5 Binary M to N Relationship Types

For each binary M:N relationship type R, create a new relation S to represent R.

Include as a foreign key attributes in S the primary keys of the relations that participates in R; their combination will form the primary key of S.


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Also include any simple attributes of the M:N relationship type (or simple component of composite attributes) as attributes of S.


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M to N Relationship Mapping

R

n

ET2

ET1

n

ET2

B

ET1

A

NO

ET2

B

ET1

A

NOET2

B

ET1

A

RA B


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Notice that we cannot represent an M:N relationship type by a single foreign key attribute in one of the participating relations – as we did for 1:1 or 1:N relationship types – because of the M:N cardinality ratio.


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Notice that we can always map 1:1 or 1:N relationships in a manner similar to M:N relationships.


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STEP 6 Multivalued Attributes

For each multivalued attribute A, create a new relation R.

The relation R will include an attribute corresponding to A, plus the primary key attribute K – as a foreign key in R – of the relation that represents the entity type or relationship type that has A as an attribute.


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STEP 6 Multivalued Attributes

The primary key of R is the combination of A and K.

If the multivalued attribute is composite, we include its simple components.


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Multivalued Attributes Mapping

F

ET-FA FET A


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STEP 7 n-ary Relationship Types Mapping

For each n-ary relationship type R, where n>2, create a new relation S to represent R.

Include as foreign key attributes in S the primary keys of the relations that represent the participating entity types.


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STEP 7 Continued

Also include any simple attributes of the n-ary relationship type (or simple components of composite attributes) as attributes of S.

The primary of S is usually a combination of all the foreign keys that reference the relations representing the participating entity types.


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STEP 7 Continued

However, if the cardinality constraints on any of entity type E participating in R is 1, then the primary key of S should not include the foreign key attribute that references relation E.


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Notes on ER-to-Relational Mapping

Relationships in ER are mapped to foreign key attributes. A single foreign key needed for 1:1 or 1:N rel

ationships A extra relation with two foreign keys neede

d for binary M:N relationship. For n-ary relationship, n>2, we need an extr

a relation with n foreign keys.


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EQUIJOIN operations are needed to materialize the relationships by combining related tuples: A single EQUIJOIN needed to materialize 1:1

or 1:N relationships. Two EQUIJOINS needed to materialize bina

ry M:N relationship N EQUIJOINS needed to materialize the full

n-ary relationship.


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Primary key of weak entity type includes key of owner relation.

Primary key of relation representing an n-ary relationship determined from the structural participation constraints.


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We create a separate relation for each multivalued attribute.

This is because the relational model does not allow multiple values (a list, or a set of values) for an attribute in a single table.


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Correspondence Between ER and Relational Models

ER Model Relational Model

Entity type “Entity” relation

1:1 or 1:N relationship type

Foreign key (or “relationship” relation)

M:N relationship type “Relationship” relation and two foreign keys

N-ary relationship type “Relationship” relation and n foreign keys

Simple attribute Attribute

Composite attribute Set of simple component attributes

Multivalued attribute Relation and foreign keys

Value set Domain

Key attribute Primary (or secondary) key

Documents

ER- Relational Mapping (Based on Chapter 7 in Fundamentals of Database Systems by Elmasri and Navathe, Ed. 4)