What is the difference b/w Primary Key and Unique Key

Question

I tried to find it out in google but not satisfactory answer is given out there. Can anybody explain the solid difference.

actually if Primary key is used to sele

Answer 1

Both are representing a unique identification to a row in a table but there is little bit difference is that

PRIMARY key does not allows NULL values

while

UNIQUE key allows only one NULL values.

that is the main difference ..