SQL - What is difference between UNIQUE and PRIMARY KEY constraints?

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What is difference between UNIQUE and PRIMARY KEY constraints?

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.

Total Answers and Comments: 11

Last Update: September 16, 2008

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Submitted by: Vijay Raj Jaiswal

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

Above answer was rated as good by the following members:
satyam_Ora, sbukka, spohilko, sumanpani

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September 20, 2005 09:43:01

Vijay Raj Jaiswal

RE: What is difference between UNIQUE and PRIMARY KEY ...

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

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4 0

Overall Rating: +4

September 28, 2005 06:09:45

kalai

RE: What is difference between UNIQUE and PRIMARY KEY ...

Primary key creates clustered index whereas unique key creates non clustered index.primary key won't allow NULL values whereas unique key allows NULL values.

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October 23, 2006 10:23:26

sugapriyaraja

RE: What is difference between UNIQUE and PRIMARY KEY ...

Primary Key: only one column in a table,not accepted null values.

Unique Key: More then one column in a table, null values accepted.

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March 01, 2007 01:23:47

bala

RE: What is difference between UNIQUE and PRIMARY KEY ...

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 one Null Value
Null value

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May 23, 2007 07:54:26

amit

RE: What is difference between UNIQUE and PRIMARY KEY ...

Primary key creates clustered index whereas unique key creates non clustered index. Primary key won't allow NULL values whereas unique key allows only one NULL values.

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July 06, 2007 05:20:48

Faizal

RE: What is difference between UNIQUE and PRIMARY KEY ...

/*
Just adding to the Primary Key Explanation
*/

Its right that there can be only one Primary Key for a Table.

But for that Primary Key we can add more than one fields.

Ex:

ALTER TABLE emp ADD CONSTRAINT PRIMARY KEY(emp_no, dept_no, phone);

By the above, all the 3 fields make a combination of a Primary Key.

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August 07, 2007 04:50:14

velsowmya

Member Since: August 2007 Contribution: 7

RE: What is difference between UNIQUE and PRIMARY KEY ...

Primary key wont accept null values and a table can have only one primary key for a column.
Unique key accepts null values.A table can have any number of unique keys.

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October 19, 2007 03:18:50

Rahul Kataria

RE: What is difference between UNIQUE and PRIMARY KEY ...

Primary key cannot take null values but Unique can take null values

simply we can say

primary key = unique + not null

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January 29, 2008 08:23:38

patilpravin_1981

Member Since: December 2007 Contribution: 29

RE: What is difference between UNIQUE and PRIMARY KEY constraints?

unique key can contains one null value
primery key can not contain null value

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May 29, 2008 08:00:14

#10

ssusmitaa

Member Since: March 2007 Contribution: 2

RE: What is difference between UNIQUE and PRIMARY KEY constraints?

A table can have only one PRIMARY KEY where as can have numerous UNIQUE KEY . A PRIMARY KEY is a unique KEY with the extra constraint that all key
columns must be defined as NOT NULL. But unique key column can contain null value only once. If UNIQUE KEY is defined on more than one column then null values can be in the table by unique combination of those columns. Another difference is that PRIMARY KEY constraints enforces clustered indexing where as UNIQUE KEY constrains are nonclustered indexes.

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