Transforming Entity-Relationship Models Into Relational Database Implementations

Two Simple Rules forTransforming an ERD into Tables

1. Every entity will be a table.

2. A relationship will be a table only if:

a. It is a many to many relationship

orb. It has its own attributes.

Placement of Foreign KeysIn a Transformation

One to Many:The primary key from “One” table is used as the foreign key in the “Many” table.

One to One:The primary key from one of the “One” table is used as the foreign key in the other “One” table.

Many to Many:The relationship becomes a “bridge” table and the primary key from each of the “Many” tables is used as a foreign key. All of the foreign keys collectively become the composite primary key of the new linking table.

Binary One to Many

PUBLISHER ( Pnum, Name, Addr )

BOOK ( ISBN, Title, Edition, Pnum )

Binary One to One

STATE ( Code, Name, Size )

GOVERNOR ( SSN, Name, BDate, Code )

or

STATE ( Code, Name, Size , SSN )

GOVERNOR ( SSN, Name, BDate )

Binary Many to Many

STUDENT ( Snum, Name, Addr )

COURSE ( Cnum, CTitle, CrHrs )

TAKES ( Snum, Cnum, Term )

Unary One to One

EMPLOYEE ( Enum, Name, SpouseEnum )

Unary One to Many

EMPLOYEE ( Enum, Name, MgrEnum )

Unary Many to Many

PART ( Pnum, Name )

ASSEMBLY ( Anum, Pnum )

Ternary

PART ( Pnum, Pname )

SUPPLIER ( Snum, SName)

WAREHOUSE ( Wnum, City )

SHIPSTO ( Pnum, Snum, WNum )

Composite Entity

STUDENT ( Snum, Name, Addr )

COURSE ( Cnum, CTitle, CrHrs )

TAKES ( Enum, Snum, Cnum, Term )

Supertype-Subtype

STUDENT ( PID, Major )

GRADUATE ( PID, Thesis )

UNDERGRADUATE ( PID, Hours )