What's the difference between a primary key and a unique key in SQL?

[beingchinmay]

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key does not allow NULLs, but unique key allows one NULL only.






Asthi jivak | Fat Cutter

[ketul]

Primary Key is used to identify a row (record) in a table,

whereas Unique-key is to prevent duplicate values in a column (with the exception of a null entry). 2. Indexing: By default SQL-engine creates Clustered Index on primary-key if not exists and Non-Clustered Index on Unique-key

[Brianvoges]

Unique Key (UK): It's a column or a group of columns that can identify a uniqueness in a row.

Primary Key (PK): It's also a column or group of columns that can identify a uniqueness in a row.


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