What is the difference between Primary Key and unique key constraint?

Question

What is the difference between Primary key And unique Key constraint?

What\'s the use of it??

Answer 1

Short version:

• From the point of view of database theory, there is none. Both are simply candidate keys.
• In practice, most DMBS like to have one "standard key", which can be used for e.g. deciding how to store data, and to tell tools and DB clients which is the best way to identify a record.

So distinguishing one unique key as the "primary key" is just an implementation convenience (but an important one).