If you are using PHP without a framework and mysql_array_assoc, there can be collisions / inconsistencies / problems in joined columns that have the same name.

If you are using PHP without a framework and mysql_array_assoc, there can be collisions / inconsistencies / problems in joined columns that have the same name. I've run into it a few times. Most decent systems don't choke in that case, but there are cases where it's simply easier to prefix column names instead of writing AS everywhere. If you're working with really long SQL queries generally it can also help make things more explicit. I don't do this usually but I have run into a couple of occasions where it would have helped out.