INSERT if NOT EXISTS Help??

[Brian Coleman]

I have a site that upon signup inserts some info into tables. Then once logged in, more info gets inserted into different tables. So what I need to do is for the second insert, check if it exists, if so UPDATE, if not INSERT. I am new to this so not sure exactly the best most efficient way to do this. Here is my SQL, any help is greatly appreciated.

When a user signs up, member_info gets updated, along with profile_info. When a user logs in, they can update info in 2009_lacrosse. But if an entry with their username and email does not exist, i need to do the Insert if NOT EXISTS.

This is my SQL that gets the record if it exists:

$sql = "SELECT * FROM member_info, 2009_lacrosse, profile_info WHERE member_info.username='$id' AND 2009_lacrosse.username='$id' AND profile_info.username='$id';";

THANKS SO MUCH IN ADVANCE!!!
Brian Coleman