Check if Username is in use

*Before someone registers, I want my

(username) field in my (_members) table to check to see if that name is already in use,

if it is in use, then I want it to tell them "Name already in use, choose another username"

if it is not in use, then they should proceed with the registration
I see the "before registration" event under Events, but I don't know which action to use and how to change the code.

Can you check to see if I have chosen the right action and if everything is right?
//**** Check if specific record exists ****

global $conn;

$strSQLExists = "select * from _members where AnyColumn='username'";

$rsExists = db_query($strSQLExists,$conn);

$data=db_fetch_array($rsExists);

if($data)

{

// if record exists do something

}

else

{

// if dont exist do something else

}
return true;
// return true if you like to proceed with registration

// return false otherwise