A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.

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Answered by: Vijay Raj Jaiswal

• Sep 20th, 2005

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Hi this is modified answer ,pls post this instead of previous which i post

Primay Key                     Unique key

-----------                  ------------------

There is only one         there may be more than 1
Primary key for           Unique Key in table
1 table

It can not contain           It Can contain Null Value
Null value

Lakshmanan

• Jun 30th, 2017

Primary key is combination of not null and unique. Primary key does not insert a null value and insert value is unique. Unique is one time accepted null value and insert a data is uniquely

ankitagujrati28

 Profile Answers by ankitagujrati28 Questions by ankitagujrati28

• Apr 30th, 2010

Unique key means we have to enter unique value but we can enter null.But in case of Primary key we should have to enter Unique + Not Null value in that particular column.

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