Why can't I use an "AS" value in the WHERE clause.

[Daevid Vincent]

I'm curious when will I be able to do something like this:

SELECT *, IF(( (unix_timestamp()-unix_timestamp(last_seen)) < 600),1,0) as
active FROM wifi_table WHERE active = 1;

It's so obnoxious, especially since I can do this:

SELECT *, IF(( (unix_timestamp()-unix_timestamp(last_seen)) < 600),1,0) as
active FROM wifi_table WHERE unix_timestamp()-unix_timestamp(last_seen) <
600;

Why do I have to do the math TWICE?!

*sigh*