Im a newbie in all this and i want to ask somethnig..
I make one progran for one garage to store the customers and their car..
All the data are being store in one database..
When the owner of the program want to update the data for one customer i want to do 2 different thinks on submit..
First i want to be sure that the programmer filed the fileds that MUST be completed to continue and after i want to check my database if the customer exist in the database..If not to show a message that the customer does not exist..
I know how to make the first step to check if the fields are completed but i dont know how to solve the second step.
Also im not sure if i can do this two thinks together by click the submit button

Can somebody help me pls?

Thnx in advance