difference between a primary key and a unique key [Resolved]

Posted by Saranya Boopathi under Sql Server on 6/26/2013 | Points: 10 | Views : 924 | Status : [Member] | Replies : 5
What's the difference between a primary key and a unique key?

Saranya Boopathi


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Posted by: Kirthiga on: 6/26/2013 [Member] Starter | Points: 50

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Primary Key:

1) Primary Key creates clustered index by default

2) Primary Key doesn't allow nulls

Unique Key:

1) Unique Key creates non-clustered index by default

2) Unique Key allows only one null value


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Posted by: Pandians on: 6/26/2013 [Member] [MVP] Silver | Points: 25

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@Kirthiga

Small Correction on Primary Key definition! "Primary key will create with Clustered only if any of the column already doesn't have any Clustered in it. Other wise Primary key will be a Non-Clustered".

We should additional Cautious whenever using the word "DEFAULT"...! Because, It may be "NOT ALWAYS"

Do you agree ?

Cheers
www.SQLServerbuddy.blogspot.com
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Posted by: Kirthiga on: 6/26/2013 [Member] Starter | Points: 25

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@Pandians

Thank u for your correction

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Posted by: Satyapriyanayak on: 6/27/2013 [Member] [MVP] Silver | Points: 25

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Primary key does not allow the null values but unique key allows one null value.
Primary key will create clustered index on column but unique key will create non-clustered index by default.

If this post helps you mark it as answer
Thanks

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Posted by: Sriramnandha on: 9/6/2013 [Member] Starter | Points: 25

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primary key restrice duplicate values and null values each table can have only one primary key..

unique key restrict duplicate values and only one null values...

hope this will help....

sriram

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