1. CS4222 Principles of Database System
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CS4222 Principles of Database System
9. ER/EER –to-Relational Mapping 2
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline ER/EER -to-Relational Mapping
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Outline
ER/EER -to-Relational Mapping
Step 5: Map Relationship Sets with Participation Constraints
Step 6: Mapping of Weak Entity Types
Step 7: Mapping of Multivalued attributes
Step 8: Mapping Specialization or Generalization
Step 9: Mapping of Shared Subclasses (Multiple Inheritance)
Step 10: Mapping of Union Types (Categories)
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3. Step 5: Map Relationship Sets with Participation Constraints
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Step 5: Map Relationship Sets with Participation Constraints
since
name
dname
ssn
sex
dno
budget 1
Manage
Department
Employees
Each department has exactly one manager
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4. Step 5: Map Relationship Sets with Participation Constraints
Combine Manage and Department to one Relation
CREATE TABLE Mg_Dept(
ssn CHAR(11) NOT NULL,
since DATE,
dno INTEGER,
dname Char(20),
budget REAL,
PRIMARY KEY (dno),
FOREIGN KEY (ssn) REFERENCES Employees (ssn)
ON DELETE RESTRICT )
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5. Step 6 Mapping of Weak Entity Types
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Step 6 Mapping of Weak Entity Types
A weak entity can be identified uniquely only by considering the primary key of another (owner) entity.
Owner entity set and weak entity set must participate in a one-to-many relationship set (1 owner, many weak entities).
Weak entity set must have total participation in this identifying relationship set.
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6. Step 6 Mapping of Weak Entity Types
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Step 6 Mapping of Weak Entity Types
For each weak entity type W in the ER schema with owner entity type E, create a relation R & include all simple attributes (or simple components of composite attributes) of W as attributes of R.
Also, include as foreign key attributes of R the primary key attribute(s) of the relation(s) that correspond to the owner entity type(s).
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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7. Step 6 Mapping of Weak Entity Types
Weak entity set and the identifying relationship set are translated into a single table.
sex
name
birthdate
dname
Dependents
Employees
ssn
Dependents_of 1 N
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8. Step 6 Mapping of Weak Entity Types
When the owner entity is deleted, all owned weak entities must also be deleted.
CREATE TABLE Dependents_Of (
ssn CHAR(11) NOT NULL
dname CHAR(20),
birthdate Date,
PRIMARY KEY (dname, ssn),
FOREIGN KEY (ssn) REFERENCES Employees(ssn)
ON DELETE CASCADE )
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9. Step 7: Mapping of Multivalued attributes
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Step 7: Mapping of Multivalued attributes
For each multivalued attribute A, create a new relation R.
This 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 of relationship type that has A as an attribute.
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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10. Step 7: Mapping of Multivalued attributes
Example: The relation DEPT_LOCATIONS is created.
The attribute DLOCATION represents the multivalued attribute LOCATIONS of DEPARTMENT, while DNUMBER-as foreign key-represents the primary key of the DEPARTMENT relation.
The primary key of R is the combination of {DNUMBER, DLOCATION}.
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11. Step 8: Mapping Specialization or Generalization
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Step 8: Mapping Specialization or Generalization
Convert each specialization with m subclasses {S1, S2,….,Sm} and generalized superclass C, where the attributes of C are {k,a1,…an} and k is the (primary) key, into relational schemas using one of the four following options:
Option 8A: Multiple relations-Superclass and subclasses
Option 8B: Multiple relations-Subclass relations only
Option 8C: Single relation with one type attribute
Option 8D: Single relation with multiple type attributes
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12. Step 8: Mapping Specialization or Generalization
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Step 8: Mapping Specialization or Generalization
Option 8A: Multiple relations-Superclass and subclasses
Create a relation L for C with attributes Attrs(L) = {k,a1,…an} and PK(L) = k.
Create a relation Li for each subclass Si, 1 < i < m, with the attributesAttrs(Li) = {k} U {attributes of Si} and PK(Li)=k.
This option works for any specialization (total or partial, disjoint of over-lapping)
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13. 6/16/2019
FIGURE 4.4 EER diagram notation for an attribute-defined specialization on JobType.
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14. 6/16/2019
FIGURE 7.4 Options for mapping specialization or generalization. (a) Mapping the EER schema in Figure 4.4 using option 8A.
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15. Step 8: Mapping Specialization or Generalization
Option 8B: Multiple relations-Subclass relations only
Create a relation Li for each subclass Si, 1 < i < m, with the attributes Attr(Li) = {attributes of Si} U {k,a1…,an} and PK(Li) = k.
This option only works for a specialization whose subclasses are total (every entity in the superclass must belong to (at least) one of the subclasses).
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16. 6/16/2019
FIGURE 4.3 Generalization. (b) Generalizing CAR and TRUCK into the superclass VEHICLE.
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17. 6/16/2019
FIGURE 7.4 Options for mapping specialization or generalization. (b) Mapping the EER schema in Figure 4.3b using option 8B.
Tonnage
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18. Step 8: Mapping Specialization or Generalization
Option 8C: Single relation with one type attribute
Create a single relation L with attributes Attrs(L) = {k,a1,…an} U {attributes of S1} U…U {attributes of Sm} U {t} and PK(L) = k.
The attribute t is called a type (or discriminating) attribute that indicates the subclass to which each tuple belongs
Works for disjoint specialization
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19. 6/16/2019
FIGURE 4.4 EER diagram notation for an attribute-defined specialization on JobType.
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20. 6/16/2019
FIGURE 7.4 Options for mapping specialization or generalization. (c) Mapping the EER schema in Figure 4.4 using option 8C.
Engtype
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21. Step 8: Mapping Specialization or Generalization
Option 8D: Single relation with multiple type attributes
Create a single relation schema L with attributes Attrs(L) = {k,a1,…an} U {attributes of S1} U…U {attributes of Sm} U {t1, t2,…,tm} and PK(L) = k.
Each ti, 1 < I < m, is a Boolean type attribute indicating whether a tuple belongs to the subclass Si.
Works for overlapping subclasses
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22. 6/16/2019
FIGURE 4.5 EER diagram notation for an overlapping (non-disjoint) specialization.
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23. 6/16/2019
FIGURE 7.4 Options for mapping specialization or generalization. (d) Mapping Figure 4.5 using option 8D with Boolean type fields Mflag and Pflag.
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24. Step 9: Mapping of Shared Subclasses (Multiple Inheritance)
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Step 9: Mapping of Shared Subclasses (Multiple Inheritance)
A shared subclass, such as STUDENT_ASSISTANT, is a subclass of several classes, indicating multiple inheritance.
These classes must all have the same key attribute
We can apply any of the options discussed in Step 8 to a shared subclass, subject to the restriction discussed in Step 8 of the mapping algorithm.
Example
both 8C and 8D are used for the shared class STUDENT_ASSISTANT.
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25. 6/16/2019
FIGURE 4.7 A specialization lattice with multiple inheritance for a UNIVERSITY database.
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26. 6/16/2019
FIGURE 7.5 Mapping the EER specialization lattice in Figure 4.6 using multiple options.
course
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27. Step 10: Mapping of Union Types (Categories)
For mapping a category whose defining superclass have different keys, it is customary to specify a new key attribute, called a surrogate key, when creating a relation to correspond to the category.
In the example below we can create a relation OWNER to correspond to the OWNER category and include any attributes of the category in this relation. The primary key of the OWNER relation is the surrogate key, which we called OwnerId.
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28. FIGURE 4.8 Two categories (union types): OWNER and REGISTERED_VEHICLE.
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29. FIGURE 9. 7 Mapping the EER categories (union types) in Figure 4
FIGURE 9.7 Mapping the EER categories (union types) in Figure 4.8 to relations.
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30. FIGURE 7.1 The ER conceptual schema diagram for the COMPANY database.
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FIGURE 7.1 The ER conceptual schema diagram for the COMPANY database.
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31. 6/16/2019
FIGURE 7.2 Result of mapping the COMPANY ER schema into a relational schema.
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32. Exercise 1 Employees Reports_To supervisor subordinate 1 sex name ssn
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33. Exercise 1 Only ONE table is needed! CREATE TABLE Employees(
ssn CHAR(11),
name CHAR(20),
sex CHAR,
ssn_super CHAR(11) NOT NULL,
PRIMARY KEY (ssn)
FOREIGN KEY (ssn_super) REFERENCES Employees(ssn)
ON DELETE restricted
ON UPDATE CASCADE)
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34. Exercise 2 1 1 name pname age ssn lot Dependents Employees Purchaser
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Exercise 2
name
Employees
ssn
lot
pname
age
Dependents 1
Purchaser
Beneficiary 1
policyid
cost
Policies
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35. Exercise 2 policyid INTEGER, cost REAL, ssn CHAR(11) NOT NULL,
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Exercise 2
The cardinality constraints allow us to combine Purchaser with Policies and Beneficiary with Dependents.
Participation constraints lead to NOT NULL constraints.
CREATE TABLE Policies (
policyid INTEGER,
cost REAL,
ssn CHAR(11) NOT NULL,
PRIMARY KEY (policyid).
FOREIGN KEY (ssn) REFERENCES Employees (ssn),
ON DELETE CASCADE )
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36. Exercise 2 pname CHAR(20), age INTEGER, policyid INTEGER,
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Exercise 2
CREATE TABLE Dependents (
pname CHAR(20),
age INTEGER,
policyid INTEGER,
PRIMARY KEY (pname, policyid).
FOREIGN KEY (policyid) REFERENCES Policies(policyid),
ON DELETE CASCADE )
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