Tuesday, 9 September 2014

Comparison of Primary key and Unique key


Compare Primary key and Unique key / Comparison of Primary key and Unique key / Differentiate Primary and Unique key



Primary Key and Unique Key Comparison

Comparison Property
Primary Key
Unique Key
How to create?
Using the keyword ‘PRIMARY KEY’
Ex. (in SQL)
attribute name type(size) PRIMARY KEY
Emp_ID CHAR(10) PRIMARY KEY
Using the keyword ‘UNIQUE’
Ex. (in SQL)
attribute name type(size) UNIQUE
Emp_ID CHAR(10) UNIQUE
Purpose
To uniquely identify records
To uniquely identify records
Null values
Cannot have NULL values
Can have at most one NULL value
Number of keys
Can have only one Primary key
Can have many unique keys
Can we use one or more attributes (columns) together to form the key?

(Composite keys)
Yes. One or more columns together can form Primary key
Yes. One or more columns together can form Unique key.
Maximum number of columns permitted in a composite key (in Oracle).
32
32
Can we Create, Add, Drop, Enable and Disable?
Yes
Yes
Is it compulsory to have these keys in a table?
No. Optional
No. Optional
Automatic creation of index tables (in Oracle)
Primary key declaration automatically creates index on the Primary key attribute.
Unique key declaration automatically creates an index on the Unique attribute.










Featured Content

Multiple choice questions in Natural Language Processing Home

MCQ in Natural Language Processing, Quiz questions with answers in NLP, Top interview questions in NLP with answers Multiple Choice Que...

All time most popular contents