Mapping ER model to Relational Schema
Or
Relational DB design using ER to Relational Mapping
Example: Company DB that we have considered for ER model
ER model for Company DB:
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ER to Relational Mapping Algorithm:
Step 1: Mapping of Regular (strong) Entity types
Step 2: Mapping of Weak Entity types
Step 3: Mapping of binary 1:1 Relationship types
Step 4: Mapping of binary 1:N Relationship types
Step 5: Mapping of binary M:N Relationship types
Step 6: Mapping of multivalued attributes
Step 7: Mapping of N-ary Relationship types
Step 1: Mapping of Regular (strong) Entity types:
For each strong entity type E in the ER schema,
1. Create a relation R that includes all simple attributes of E
2. If attribute is composite, then include only simple component
attributes.
3. Choose one of the key attributes of E as primary key for R
4. If the chosen key of E is a composite, then set of simple attributes
that forms it will together form the primary key of R
The result after this step is shown below:
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Step 2: Mapping of Weak Entity types:
For each weak entity type W in the ER schema with owner entity type E,
1. Create a relation R that includes all simple attributes of W
2. If attribute is composite, then include only simple component
attributes.
3. Include the primary key of owner entity type E as foreign key
attribute of R
4. The primary key of R is the combination of primary key(s) of
owner(s) and partial keys of weak entity type W, if any.
The result after this step is shown below:
Note: we have renamed the primary key of owner entity type ssn into
Essn. But it is not necessary to rename.
Step 3: Mapping of binary 1:1 Relationship types:
For each binary 1:1 relationship type R in the ER schema, identify the
relations S and T
Foreign key approach – most useful and should be followed
a. Choose one relation as S (better to choose entity type with
total participation in R as S)
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b. Include primary key of T as foreign key in S
c. Include simple attributes (or simple components of composite
attributes) of the 1:1 relationship type as attributes of S
In our example ER schema, 1:1 relationship is ‘manages’
S – Department; T – Employee
The result after this step is shown below:
Step 4: Mapping of binary 1:N Relationship types:
For each binary 1:N relationship type R,
1. Identify relation S (S relation will be the entity type at the N-side
of the relationship type)
2. Include primary key of T as foreign key in S
3. Include simple attributes (or simple components of composite
attributes) of the 1:N relationship type as attributes of S
In our example ER schema, 1:N relationships are ‘works_for’,
‘supervision’, ‘controls’
Works_for: S – Employee; T – Department
Supervision: S – Employee; T – Employee
Controls: S – Project; T – Department
The result after this step is shown below:
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Step 5: Mapping of binary M:N Relationship types:
For each binary relation of M:N relationship type,
1. Create new relation S
2. Include primary keys of participating relations as foreign key in S
3. Primary key combination of participating relations form the
primary key of S
4. Include any simple attributes or simple components of composite
attributes of M:N relationship type as attributes of S
In our example ER schema, M:N relationship is ‘works_on’ between
project and employee.
The result after this step is shown below:
Step 6: Mapping of multivalued attributes:
For each multivalued attribute A,
1. Create new relation R
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2. R contains an attribute corresponding to A and the primary key
attribute K of the relation or relationship type that has A as
attribute
3. K becomes the foreign key
4. Primary key of R is the combination of A and K
In our example ER schema, multivalued attribute is locations in
Department relation.
The result after this step is shown below:
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Company relational DB schema obtained with step 1 through 6:
Note:
There is no n-ary relationship type in our example.
Refer the above figure for foreign key reference representation.
Step 7: Mapping of N-ary Relationship types:
For each n-ary relationship type R, where n>2,
1. Create new relation S
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2. Include the primary key of relations that represents the
participating entity types as foreign key
3. Include any simple attributes or simple components of composite
attributes of N-ary relationship type as attributes of S
4. Combination of all foreign keys forms the primary key of S
Example:
Note:
If cardinality constraint of any of the entity type participating on R
is 1, then primary key of S should not include the foreign key
attribute that references the corresponding relation
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