Chapter 9 [Relational Database Design by ER- and EERR-to-Relational Mapping]

Goals during Mapping

Preserve all information (that includes all attributes) Maintain the constraints to the extent possible (Relational Model cannot preserve all contstraints- e.g., max cardinality ratio such as 1:10 in ER; exhaustive classification into subtypes, e.g., STUDENTS are specialized into Domestic and Foreign) Minimize null values

Step 8: Options for Mapping Specialization or Generalization.

1. 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

Step 3: Mapping of Binary 1:1 Relation Types

1. For each binary 1:1 relationship type 'R' in ER schema, identify the relations 'S' and 'T' that correspond to the entity types participating in 'R' and there are 3 possible approaches: -Foreign Key approach -Merged Relation option -Cross-reference or relationship relation option

Step 6: Mapping of Multivalued attributes.

1. For each multivalued attribute A, create a new relation R. 2. 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. 3. The primary key of R is the combination of A and K. If the multivalued attribute is composite, we include its simple components.

Step 7: Mapping of N-ary Relationship Types.

1. For each n-ary relationship type R, where n>2, create a new relationship S to represent R. 2. Include as foreign key attributes in S the primary keys of the relations that represent the participating entity types. 3. Also include any simple attributes of the n-ary relationship type (or simple components of composite attributes) as attributes of S.

Step 4: Mapping of Binary 1:N Relationship Types

1. 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 2. Include as foreign key in 'S' the primary key of the relation 'T' that represents the other entity type participating in 'R'. 3. Include any simple attributes of the 1:N relation type as attributes of 'S'

Step 5: Mapping of Binary M:N Relationship Types

1. For each regular binary M:N relationship type R, create a new relation S to represent R. This is a relationship relation. 2. 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. 3. Also include any simple attributes of the M:N relationship type (or simple components of composite attributes) as attributes of S.

Step 1: Mapping of Regular Entity Types

1. For each regular(strong) entity type 'E' in the ER schema, CREATE a relation 'R' that includes ALL simple attributes of 'E' 2. 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 from the primary key of 'R') -If there are multiple keys, the information describing the attributes that form each additional key is kept in order to keep track of additional keys of 'R'

Step 2: Mapping of Weak Entity Types

1. For each weak entity type 'W' in ER schema with owner entity type 'E' create a relation 'R' and include ALL simple attributes (or simple components of composite attributes) of 'W' as attributes of 'R' 2. Also, include as foreign key attributes of 'R' the primary key attributes(s) of the relation(s) that correspond to the owner entity type(s) 3. 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 (if there is a weak entity type E1, then E1 should be mapped before E2 to determine its primary key first)

Step 9: Mapping of Union Types (Categories).

1. 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.

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).

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.

Step 3: Mapping of Binary 1:1 Relation Types Cross-Reference or Relationship Relation Option

Cross-Reference or Relationship Relation (3 relations) Option: 1. The third alternative is 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.

Example of Step 4: Mapping of Binary 1:N Relationship Types

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. An alternative approach is to use a Relationship relation (cross referencing relation) - this is rarely done.

Example of Step 2: Mapping of Weak Entity Types

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.

Example of Step 5: Mapping of Binary M:N Relationship Types

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}.

Example of Step 6: 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}.

Example of Step 7: Mapping of N-ary Relationship Types

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}

Step 3: Mapping of Binary 1:1 Relation Types The Foreign Key Approach

Foreign Key (2 relations) Approach: 1. 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' [include all simple attributes of 1:1 relationship type 'R' as attributes 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.

Step 3: Mapping of Binary 1:1 Relation Types Merged Relation Option

Merged relation (1 Relation) option: 1. An alternative mapping of a 1:1 relationship type is possible by merging the two entity types and the relationship into a single Relation. [only appropriate when both participations are total.]

ER MODEL Simple Attribute

RELATIONAL MODEL Attribute

ER MODEL Value Set

RELATIONAL MODEL Domain

ER MODEL Entity Type

RELATIONAL MODEL Entity Relation

ER MODEL 1:1 or 1:N relationship type

RELATIONAL MODEL Foreign Key (or relationship relation)

ER MODEL Key Attribute

RELATIONAL MODEL Primary (or secondary) key

ER MODEL Multivalued Attribute

RELATIONAL MODEL Relation and Foreign Key

ER MODEL N-ary relationship type

RELATIONAL MODEL Relationship relation and 'n' foreign keys

ER MODEL M:N relationship type

RELATIONAL MODEL Relationship relation and two foreign keys

ER MODEL Composite Attribute

RELATIONAL MODEL Set of simple component attributes

REMEMBER TO LOOK AT VISUAL EXAMPLES OF MAPPING ON POWERPOINT!

REMEMBER TO LOOK AT VISUAL EXAMPLES OF MAPPING ON POWERPOINT!

7 Steps of 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.

9 Steps of Mapping EER Model Constructs to Relations

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. Step 8: Options for Mapping Specialization or Generalization. Step 9: Mapping of Union Types (Categories).

Example of STEP 1: Mapping of Regular Entity Types

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.