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

A primary key is just any one candidate key. In principle primary keys are not different from any other candidate key because all keys are equal in the relational model.

SQL however has two different syntax for implementing candidate keys: the PRIMARY KEY constraint and the UNIQUE constraint (on non-nullable columns of course). In practice they achieve exactly the same thing except for the essentially useless restriction that a PRIMARY KEY can only be used once per table whereas a UNIQUE constraint can be used multiple times.

So there is no fundamental "use" for the PRIMARY KEY constraint. It is redundant and could easily be ignored or dropped from the language altogether. However, many people find it convenient to single out one particular key per table as having special significance. There is a very widely observed convention that keys designated with PRIMARY KEY are used for foreign key references, although this is entirely optional.