HAVING confusion..

Hello,

I'm using postgresql 6.4.2 and i'm having a problem with the
"HAVING" clause..

Basically I do

SELECT u_id,expire_date FROM users HAVING expire_date >= (NOW())

expire_date is of timestamp data type.

and it keeps failing with
ERROR: This is not a valid having query!

even doing something simple like
SELECT u_id from users having u_id > 5

fails with the same error..

select sell_by_date,item_num FROM sell,users,i
tem WHERE sold_by = u_id AND sell_item = item_num AND category_id =
'$parameter'
ORDER BY register_date DESC HAVING sell_by_date >= (NOW())

basically I want to do the above that works with great with mysql
, solid and etc..