Enhanced E-R model

2

Keys are special fields that serve two main purposes:◦ Primary keys are unique identifiers of the relation in

question. Examples include employee numbers, social security numbers, etc. This is how we can guarantee that all rows are unique

◦ Foreign keys are identifiers that enable a dependent relation (on the many side of a relationship) to refer to its parent relation (on the one side of the relationship)

Keys can be simple (a single field) or composite (more than one field)

Keys usually are used as indexes to speed up the response to user queries

Key Fields

3

Primary Key

Foreign Key (implements 1:N relationship between customer and order)

Combined, these are a composite primary key (uniquely identifies the order line)…individually they are foreign keys (implement M:N relationship between order and product)

4

Domain Constraints◦ Allowable values for an attribute.

Entity Integrity◦ No primary key attribute may be null. All primary

key fields MUST have data Action Assertions

◦ Business rules.

Integrity Constraints

5

Mapping Regular Entities to Relations 1. Simple attributes: E-R attributes map directly

onto the relation2. Composite attributes: Use only their simple,

component attributes 3. Multivalued Attribute - Becomes a separate

relation with a foreign key taken from the superior entity

Transforming EER Diagrams into Relations

6

(a) CUSTOMER entity type with simple attributes

Figure 5-8: Mapping a regular entity

(b) CUSTOMER relation

7

(a) CUSTOMER entity type with composite attribute

Mapping a composite attribute

(b) CUSTOMER relation with address detail

8

Figure 5-10: Mapping a multivalued attribute

1–to–many relationship between original entity and new relation

(a)

Multivalued attribute becomes a separate relation with foreign key

(b)

9

Mapping Weak Entities◦Becomes a separate relation with a foreign key taken from the superior entity

◦Primary key composed of: Partial identifier of weak entity Primary key of identifying relation

(strong entity)

Transforming EER Diagrams into Relations (cont.)

10

11

NOTE: the domain constraint for the foreign key should NOT allow null value if DEPENDENT is a weak entity

Foreign key

Composite primary key

12

Mapping Unary Relationships◦ One-to-Many - Recursive foreign key in the same

relation◦ Many-to-Many - Two relations: One for the entity type One for an associative relation in which

the primary key has two attributes, both taken from the primary key of the entity

Transforming EER Diagrams into Relations (cont.)

13

Mapping a unary 1:N relationship

(a) EMPLOYEE entity with Manages relationship

(b) EMPLOYEE relation with recursive foreign key

14

Figure 5-18: Mapping a unary M:N relationship

(a) Bill-of-materials relationships (M:N)

(b) ITEM and COMPONENT relations

MAPPING TERNARY RELATION

16

Mapping the ternary relationship

Remember that the primary key MUST be

unique

Supertype:A sub grouping of the entities in an entity type which has attributes that are distinct from those in other sub groupings

Supertype: An generic entity type that has a relationship with one or more subtypes

Supertypes and Subtypes

Employee supertype with three subtypes

Relationships at the supertype level indicate that all subtypes will participate in the relationship

The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level

Relationships and Subtypes

Supertype/subtype relationships in a hospital

Both outpatients and resident patients are cared for by a responsible physician

Only resident patients are assigned to a bed

22

Supertype/subtype relationships

23

Mapping Supertype/subtype relationships to relations

These are implemented as one-to-one relationships

Generalization: The process of defining a more general entity type from a set of more specialized entity types. BOTTOM-UP

Specialization: The process of defining one or more subtypes of the supertype, and forming supertype/subtype relationships. TOP-DOWN

Generalization and Specialization

Example of generalizationThree entity types: CAR, TRUCK, and MOTORCYCLE

All these types of vehicles have common attributes

Generalization to VEHICLE supertypeSo we put the shared attributes in a supertype

Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype◦Total Specialization Rule: Yes (double line)

◦Partial Specialization Rule: No (single line)

Constraints in Supertype/ Completeness Constraint

Examples of completeness constraints

Total specialization rule

A patient must be either an outpatient or a resident patient

Partial specialization rule

Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes◦ Disjoint Rule: An instance of the supertype can be

only ONE of the subtypes◦ Overlap Rule: An instance of the supertype could

be more than one of the subtypes

Constraints in Supertype/ Disjointness constraint

Introducing a subtype discriminator (disjoint rule)

A simple attribute with different possible values indicating the subtype

Subtype discriminator (overlap rule)

A composite attribute with sub-attributes indicating “yes” or “no” to determine whether it is of each subtype