IF Statements

I have a page for listing the contents of a table from my db. Being client.php?action=list As the action variable also allows for adding extra to the table, etc. Once I use ?action=list I want to be able to click on a single record and have it give me the extended info. so client.php?action=list&clientcode=(the clients id in the db) My problem is working out how to evaluate this new part. I have normal If statements for the listing, adding, etc. But how can I get it to check if clientcode is present. If clientcode is not present I want it to load the list page as normal. But if the clientcode is present then it is to load something else. How do I structure my IF statement?