1ER ModelingBUAD/American University

Mapping ER modeling to Relationships

2ER ModelingBUAD/American University

Review of Relation Properties

• Relation

– Every relation has a unique name.

– Every attribute value is atomic.

– Every row is unique.

– Attributes in tables have unique names.

– The order of the columns is irrelevant.

– The order of the rows is irrelevant.

3ER ModelingBUAD/American University

Review of Relational Keys and Structures

• Primary Key

• Candidate Key

• Composite Key

• Foreign Key: an attribute (or a composite attribute) in a relation that serves as the primary key of another relation– One-to-Many Relationship– Many-to-Many Relationship

4ER ModelingBUAD/American University

Schema for four relations (Pine Valley Furniture)

5ER ModelingBUAD/American University

Review of Integrity Constraints• Domain Constraints

– Allowable values for an attribute.

• Entity Integrity

– No primary key attribute may be null.

• Referential Integrity: maintains consistency among related relations

– Foreign Key value in one relation must match a primary key value in other relation

– For example: Delete Rules

• Restrict, Cascade, Set-to-Null

• Operational Constraints: Business rules

6ER ModelingBUAD/American University

Referential integrity constraints (Pine Valley Furniture)

7ER ModelingBUAD/American University

Well-Structured Relations

• Is a relation that contains minimal redundancy and allows users to insert, modify, and delete the rows in a table without errors or inconsistencies

8ER ModelingBUAD/American University

Transforming E-R Diagrams Into Relations

3. Map Binary Relationships– One-to-Many - Primary key on the one side becomes a

foreign key on the many side

– Many-to-Many - Create a new relation with the primary keys of the two entities as its primary key

– One-to-One - Primary key on the mandatory side becomes a foreign key on the optional side

9ER ModelingBUAD/American University

Translating an ER diagram into Relationships (one to many)

• Create one table for each entity

• For each entity that is only at the “one” side a one to many relationship (not many end), create a single column primary (use an arbitrary unique number if no natural key exists)

• For each entity that is at the many side of a one to many relationship, use the primary key of the parent (one side) in the table as the foreign key

• Entity at the many side of one or more relationship has a natural key, use that single column as the primary key. Else, concatenate the primary key of the one side with any columns needed for uniqueness

10ER ModelingBUAD/American University

Example of mapping a 1:M relationship

(a) Relationship between customers and orders

(0, M)

1

11ER ModelingBUAD/American University

(b) Mapping the relationship

12ER ModelingBUAD/American University

Example of mapping an M:N relationship

(a) Requests relationship (M:N)

(0, M) (1, M)

13ER ModelingBUAD/American University

(b) Three resulting relations

14ER ModelingBUAD/American University

Mapping a binary 1:1 relationship

(a) Binary 1:1 relationship

(0, 1)

1

15ER ModelingBUAD/American University

(b) Resulting relations

16ER ModelingBUAD/American University

Transforming E-R Diagrams Into Relations

4. Map Associative Entities– Primary Keys

• Default primary key for the association relation is the primary keys of the two entities

17ER ModelingBUAD/American University

Mapping an associative entity with an identifier

(a) Associative entity (SHIPMENT)

18ER ModelingBUAD/American University

(b) Three relations

19ER ModelingBUAD/American University

Mapping Supertype/subtype relationships to relations

20ER ModelingBUAD/American University

Map Composite and Multi-valued Entities to Relations

– Composite attributes: Use only their simple, component attributes

– Multi-valued Attribute - Becomes a separate relation with a foreign key taken from the superior entity

21ER ModelingBUAD/American University

(a) CUSTOMER entity type with composite attribute

Mapping a composite attribute