PRIMARY KEY IS A FIELD WHICH IS USED TO IDENTIFY THE TABLE 
AND THESE CANNOT BE CHANGED, IF IT IS CHANGED THE DATA 
INTEGRITY WILL GET LOST.

UNIQUE KEY IS A GROUP OR A SINGLE FIELD WHICH IS USED TO 
REPRESENT THE NUMERIC FIELDS WHICH DOESNT CHANGE.
THE GROUP OF NUMERIC FIELDS IS THE UNIQUE KEY.

PRIMARY KEY IS USED FOR DATABASE TABLES WHERE AS UNIQUE KEY 
IS USED FOR INTERNAL TABLES.

ABOVE ITS WRONG FOR UNIQUE KEY, SORRY.

UNIQUE KEY REFERS TO THE NON-NUMERIC KEY .

Primary key does not allow the null values but where as in 
unique key accepts the null values.

Fields in UNIQUE CONSTRAINTs can be NULL. They cannot with PRIMARY KEYs.
You can define more than one UNIQUE KEY in a table.
Because of this, the behaviour of UNIQUE CONSTRAINTs
is very different from PRIMARY KEY constraints.
As long as one of the fields in a UNIQUE CONSTRAINT is NULL
the constraint itself does not apply and you can set the other fields
to whatever you want without getting an error.