Do it exist?

PostgreSQL Version 7.3.0

I hope I am not Cross Posting..with psql-novice..If so..sorry..

We have a situation where in our application the user provides the Order
Number for a delivery document.

When the Order Number is entered we need to determine if the Order Number
already exists and raise a flag if it does exist.

In another language I would do: find first Order where Order.Order_num = "1111". if avaialble Order then it_exists =
true.

In PostgreSQL ?:   SELECT DISTINCT Order_num from Order_header where Order_num = '1111';
   Is this close? If so, how to raise flage that row exists?


How we do somthing like this in SQL?


-- 
Hal Davison
Davison Consulting