When to use Primary key and Unique key

Vithal Wadje

13y2.4k

When to use Primary key and Unique key ?

11 Replies

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Neha Sharma13y ago
Accepted answer
unique keys accept null values whereas primary keys not
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Ashutosh Sharma13y ago
Neha !! Is Vishnujeet Kumar correct ?? I think to be refference key it may be either PK or Unique Key.
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Neha Sharma13y ago
ALL THE BEST...Vithal
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Vishnujeet Kumar13y ago
One more important scenario, if you want set references as foreign key ,then you have only one option first create primary key .
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Vithal Wadje13y ago
yes neha its correct,thanx for your effort,i will see it practically
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satish babu13y ago
Actually we can use these 2 constraints for entering unique values.But another advantage is when your are using the Primary key defaultly clustered index is created.When you are using unique constraints no indexes are created implicitly you can create index Explicitly.
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Neha Sharma13y ago
hi vithal, that's fine but your reply is hidden here in every answer you need to understand your situation while development. as per i consider i used uniqe key concept whenever i wantunique value with null value as well or when i want to generate id for each tables i prefer primary key always. hope now fine.
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Vithal Wadje13y ago
thanks for all reply...i alreday knows the diffrence between these two but which situationi will use it...

my question is that in which situation i need to allow one null value that is which situation i need a uniqe key to apply...

any real time example you can explain
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Jignesh Trivedi13y ago
hi,

Both Primary and Unique Keys are enforce uniqueness of the column.
Primary Key creates clustered index by default and doesn't allow nulls.
Unique Key creates non-clustered index by default and allows only one null value.

you can choose any one from PK and Unique key but as you know table contain only one PK so if you required Uniquness on other column also then you may use Unique key on that column.

hope this will help you.
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sathish kumar13y ago
   both primary key and unique key are used to avoid being inserted duplicate values.

   When u want to have one NULL values in ur data u can use UNIQUE KEY . Because Unique key can accept one NULL value.

But in t case of Primay key, it doesnt accept any NULL value. So when u dont want being entered NULL values in ur table u can use PRIMARY KEY..

   Hope this will be helpful for u.....
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Satyapriya Nayak13y ago
http://www.1keydata.com/sql/sql-primary-key.html

http://www.1keydata.com/sql/sql-unique.html
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