Question

Differentiate between primary key and candidate key.

Answer 1

Basis of Difference	Primary Key	Candidate Key
Meaning	The key selected to uniquely identify rows.	Any key eligible to be selected as the primary key.
Number in a table	Only one primary key is selected.	A table may have one or more candidate keys.
NULL values	Cannot contain NULL values.	Candidate keys must also uniquely identify records and are normally not null.
Selection	Chosen from the candidate keys.	May or may not be selected as the primary key.
Example	AdNo selected as the key.	Both AdNo and Name may be candidate keys.