surrogate key is a System generated Key ( Artificial 
key ),Every join between dimension tables and fact tables 
in a data warehouse environment should be based on 
surrogate keys, not natural keys.
There are still more reasons to use surrogate keys. One of 
the most important is the need to encode uncertain 
knowledge. You may need to supply a customer key to 
represent a transaction, but perhaps you donýt know for 
certain who the customer is. This would be a common 
occurrence in a retail situation where cash transactions 
are anonymous, like most grocery stores.
 What is the customer key for the anonymous customer? 
Perhaps you have introduced  a special key that stands for 
this anonymous customer. This is politely referred 
to as a "hack."

surrogate key is used to avoid the critical column.critical 
column means the column which makes the analysation or data 
wrong for example assume one customer table 
cust_id(p.k)  cust_name  amount  loc
1              jones       1000   hyd
he purchase the product in hyd by 1000/- if he again 
purchase by 2000/- in pune not in hyd.then we have to 
create another record in that table with same cust_id but 
here cust_id is having primary key so we are unable to 
create a new record for suppose if u update the record u 
got the wrong information as 
1    jones  3000  pune 
so eliminating this we are maintian the another column like
cust_no as surrogate key (unique value)and removes p.k in 
cust_id.surrogate key is like an index.

IT IS UNIQUE KEY AND ITS USED FOR COMPARING THE EMPLOYEE DETAILS