ch07-Relational Database Design by ER- and EERR-to-Relational Mapping.pdf

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Copyright © 2007 Ramez Elmasri and Shamkant B. Navathe

Chapter 7
Relational Database Design by ER-
and EERR-to-Relational Mapping

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Chapter Outline
 ER-to-Relational Mapping Algorithm
 Step 1: Mapping of Regular Entity Types
 Step 2: Mapping of Weak Entity Types
 Step 3: Mapping of Binary 1:1 Relation 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.
 Mapping EER Model Constructs to Relations
 Step 8: Options for Mapping Specialization or Generalization.
 Step 9: Mapping of Union Types (Categories).

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ER-to-Relational Mapping Algorithm
 Step 1: Mapping of Regular Entity Types.
 For each regular (strong) entity type E in the ER schema,
create a relation R that includes all the simple attributes of
E.
 Choose one of the key attributes of E as the primary key for
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.
 Example: We create the relations EMPLOYEE,
DEPARTMENT, and PROJECT in the relational schema
corresponding to the regular entities in the ER diagram.
 SSN, DNUMBER, and PNUMBER are the primary keys for
the relations EMPLOYEE, DEPARTMENT, and PROJECT
as shown.

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FIGURE 7.1
The ER conceptual schema diagram for the COMPANY database.

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FIGURE 7.2
Result of mapping the COMPANY ER schema into a relational schema.

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ER-to-Relational Mapping Algorithm (contd.)
 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.
 Example: Create the relation DEPENDENT in this step to
correspond to the weak entity type DEPENDENT.
 Include the primary key SSN of the EMPLOYEE relation as a
foreign key attribute of DEPENDENT (renamed to ESSN).
 The primary key of the DEPENDENT relation is the combination
{ESSN, DEPENDENT_NAME} because DEPENDENT_NAME is
the partial key of DEPENDENT.

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 For each binary 1:1 relationship type R in the ER schema, identify the
relations S and T that correspond to the entity types participating in R.
 There are three possible approaches:
1. Foreign Key approach: Choose one of the relations-say S-and include a
foreign key in S the primary key of T. It is better to choose an entity type
with total participation in R in the role of S.

Example: 1:1 relation MANAGES is mapped by choosing the participating
entity type DEPARTMENT to serve in the role of S, because its participation
in the MANAGES relationship type is total.
2. Merged relation option: An alternate mapping of a 1:1 relationship type
is possible by merging the two entity types and the relationship into a
single relation. This may be appropriate when both participations are
total.
3. Cross-reference or relationship relation option: The third alternative
is to set up a third relation R for the purpose of cross-referencing the
primary keys of the two relations S and T representing the entity types.

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 For each regular binary 1:N relationship type R, identify the
relation S that represent the participating entity type at the
N-side of the relationship type.
 Include as foreign key in S the primary key of the relation T
that represents the other entity type participating in R.
 Include any simple attributes of the 1:N relation type as
attributes of S.
 Example: 1:N relationship types WORKS_FOR,
CONTROLS, and SUPERVISION in the figure.
 For WORKS_FOR we include the primary key DNUMBER
of the DEPARTMENT relation as foreign key in the
EMPLOYEE relation and call it DNO.

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 For each regular binary M:N relationship type R, 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; their
combination will form the primary key of S.
 Also include any simple attributes of the M:N relationship type (or
simple components of composite attributes) as attributes of S.
 Example: The M:N relationship type WORKS_ON from the
ER diagram is mapped by creating a relation WORKS_ON
in the relational database schema.
 The primary keys of the PROJECT and EMPLOYEE relations are
included as foreign keys in WORKS_ON and renamed PNO and
ESSN, respectively.
 Attribute HOURS in WORKS_ON represents the HOURS attribute of
the relation type. The primary key of the WORKS_ON relation is the
combination of the foreign key attributes {ESSN, PNO}.

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 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.
 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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 For each n-ary relationship type R, where n>2, create a new
relationship S to represent R.
 Include as foreign key attributes in S the primary keys of the
relations that represent the participating entity types.
 Also include any simple attributes of the n-ary relationship
type (or simple components of composite attributes) as
attributes of S.
 Example: The relationship type SUPPY in the ER on the
next slide.
 This can be mapped to the relation SUPPLY shown in the
relational schema, whose primary key is the combination of the
three foreign keys {SNAME, PARTNO, PROJNAME}

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FIGURE 4.11
Ternary relationship types. (a) The SUPPLY relationship.

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FIGURE 7.3
Mapping the n-ary relationship type SUPPLY from Figure 4.11a.

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Summary of Mapping constructs and
constraints
Table 7.1 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 key
Value set Domain
Key attribute Primary (or secondary) key

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Mapping EER Model Constructs to
Relations
 Step8: Options for 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:
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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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Relations
 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).
 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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FIGURE 4.4
EER diagram notation for an attribute-defined specialization on JobType.

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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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FIGURE 4.3
Generalization. (b) Generalizing CAR and TRUCK into the superclass VEHICLE.

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FIGURE 7.4
(b) Mapping the EER schema in Figure 4.3b using option 8B.

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Mapping EER Model Constructs to Relations
(contd.)
 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
 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.

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FIGURE 4.4
EER diagram notation for an attribute-defined specialization on JobType.

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FIGURE 7.4
(c) Mapping the EER schema in Figure 4.4 using option 8C.

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FIGURE 4.5
EER diagram notation for an overlapping (non-disjoint) specialization.

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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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Relations (contd.)
 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;
otherwise, the shared subclass would be modeled as a
category.
 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. Below both 8C and 8D are used
for the shared class STUDENT_ASSISTANT.

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FIGURE 4.7
A specialization lattice with multiple inheritance for a UNIVERSITY
database.

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FIGURE 7.5
Mapping the EER specialization lattice in Figure 4.6 using multiple
options.

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Relations (contd.)
 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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FIGURE 4.8
Two categories (union types): OWNER and REGISTERED_VEHICLE.

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FIGURE 7.6
Mapping the EER categories (union types) in Figure 4.7 to relations.

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Mapping Exercise
Exercise 7.4.
FIGURE 7.7
An ER schema for a
SHIP_TRACKING
database.

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Chapter Summary
 ER-to-Relational Mapping Algorithm
 Step 1: Mapping of Regular Entity Types
 Mapping EER Model Constructs to Relations
 Step 8: Options for Mapping Specialization or Generalization.

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