Info comparison bet forms and database

Hi, I am intending to find out some information or tips on the following :

I had a web and a database. All the individual data are input in the system database B.

The client will submit an online web form with info promptly filled up to my system Database A. I will need to compare it to the default saved client's database in Database B.

But instead of manually go through the checking of the fields, I wish to use a program or write a program to do the auto checking and alert me of any mismatched information between the 2 database. Could you kindly assist me on how to go about that or where can I find such information?

Thank You very much sincere and urgent!