Primary Doesn't allow any dublicate value and null value. But unique allow only one null value. simple and easy
Hi,
You can have multiple unique keys in a table but can have only one Primary Key.Primary key and unique key does not allow enter duplicate data.Primary key doesn't allow null value.Unique key allows only one null value.
Both primary key and unique enforce uniqueness of the column on whichthey are defined. But by default primary key creates a clustered indexon the column, where are unique creates a nonclustered index bydefault. Another major difference is that, primary key doesn't allowNULLs, but unique key allows one NULL only.