primary key and unique key in sql which is better

The only difference is unique can contain one null value where as primary key cannot contain any null.

Unique Example:
1
2
NULL
3
4
NULL (This will throw constraint violation error as it will be duplicate)
4 (This will also throw constraint violation error as it will be duplicate)

Primary key Example:
1
2
NULL (This will throw constraint violation error)
3
4
4 (This will also throw constraint violation error as it will be duplicate)

And it is always based on your requirement . Please read here for more info

http://www.programmerinterview.com/index.php/database-sql/differences-between-primary-and-foreign-keys/

Please accept and up vote
Kindest Regards
Sibeesh
http://sibeeshpassion.com/

2 Replies

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Ranjit Powar11y ago
If you want to store null value in your column then use unique key. But only one null value is stored.

If you use primary key then you are not able to insert null value.
Sibeesh Venu11y ago
The only difference is unique can contain one null value where as primary key cannot contain any null.

Unique Example:
1
2
NULL
3
4
NULL (This will throw constraint violation error as it will be duplicate)
4 (This will also throw constraint violation error as it will be duplicate)

Primary key Example:
1
2
NULL (This will throw constraint violation error)
3
4
4 (This will also throw constraint violation error as it will be duplicate)

And it is always based on your requirement . Please read here for more info

http://www.programmerinterview.com/index.php/database-sql/differences-between-primary-and-foreign-keys/

Please accept and up vote
Kindest Regards
Sibeesh
http://sibeeshpassion.com/