Database Management Systems (dbms)

Key Types of a Relation..

• Primary key
• One of the relation candidate key is selected as primary key.
• The other candidate keys are called alternate keys.
• E.g.Consider the following relation schema Department (d#, dname, location )
• Assumption : d# and dname are unique
• Primary key: d#
• Alternate key: dname

Key Types of a Relation..

• Foreign key
• A foreign key is an attribute in one relation that matches the primary key attribute of another relation.
• E.g.Consider the following relation schemas Department (d#, dname, location ) Project (P#, desc, d#, Start_date)
• Foreign key: d# is foreign key in Project schema

Dept_No	Dept_Name	Phone
D01	Construction	0415670045
D02	Finance	0412222652
D03	Personal

FK
Project
Department

Prj_No	Prj_Name	Location	Dept
P01	ABC Auditing	Matara	D02
P02	TRC Budgeting	Colombo	D02
P03	Ground Construction	Galle	D01

Eg: Foreign key...
PK
PK