ER- Relational Mapping

(Based on Chapter 9 in Fundamentals of Database Systems by Elmasri and Navathe, Ed. 3)

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General GuidelinesEntity - RelationN:1 Relationship - Foreign KeyM:N Relationship - Relation

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ER-to-Relational Mapping Algorithm

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

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STEP 1Strong Entity TypesFor 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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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

ET AET

BA

B

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Composite Attributes Mapping

ETBA D EC

E

D

ETA B

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STEP 2Weak Entity TypesFor 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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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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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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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

ET1A

B

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STEP 3 Binary N:1Relationship TypesFor each regular binary N:1

relationship type R, identify the relations S that represents the N-side of the relationship type.

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Include as foreign key in S the primary key of the 1-side of 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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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

ET1A

ET2B A

ET2B

ET1A

NOA

B

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STEP 4 Binary 1:1 Relationship TypesFor each binary 1:1 relationship type

R in the ER schema, and S and T, the relations that participate in RChoose 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

ET2B

ET1A B

ET1A

ET2B A

- or -

A

B

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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

ET1A

ET2B AR

1

ET2

ET1

1

A

B

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STEP 5 Binary M to N Relationship TypesFor 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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STEP 5 Binary M to N Relationship TypesAlso 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

ET2B

ET1A

NO

ET2B

ET1A

NOET2

B

ET1A

RA B

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STEP 5 Binary M to N Relationship TypesNotice 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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STEP 5 Binary M to N Relationship TypesNotice 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 AttributesFor 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 Multi-valued AttributesThe 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 TypesFor 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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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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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 MappingRelationships in ER are mapped to foreign key attributes.

A single foreign key needed for 1:1 or 1:N relationshipsA extra relation with two foreign keys needed for binary M:N relationship.For n-ary relationship, n>2, we need an extra relation with n foreign keys.

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Notes on ER-to-Relational MappingEQUIJOIN 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 binary M:N relationshipN EQUIJOINS needed to materialize the full n-ary relationship.

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Notes on ER-to-Relational MappingPrimary key of weak entity type includes key of owner relation.Primary key of relation representing an n-ary relationship determined by the structural participation constraints.

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Notes on ER-to-Relational MappingWe 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 ModelsER Model Relational ModelEntity type “Entity” relation1:1 or 1:N relationship type Foreign key (or “relationship” relation)M:N relationship type “Relationship” relation and two foreign

keysN-ary relationship type “Relationship” relation and n foreign keysSimple attribute Attribute Composite attribute Set of simple component attributesMultivalued attribute Relation and foreign keysValue set DomainKey attribute Primary (or secondary) key