Select One Row From A View

I am using some UPS shipping software that imports shipping information based on an order number that the user enters and we are having performance problems. In the software's configuration you choose one database object (namely a table or view) to get the information from. I have created myView that selects from multiple tables, lookup tables, etc. I must use a view because the software only has the ability to add a " and <importKeyThatIHaveDefined> = <valueEnteredByUser>". For instance " and order_number = 123". It cannot join multiple tables, etc. The address_field1 info is the same for each record with the same order_number so I can just get the first row (or any row with order_number = <enteredOrderNumber> for that matter).

When I first looked at this it was created as something like this:

select order_number, line_number, order_number || '' || line_number as ol, address_field1 from order;Keep in mind that there are more than just this table and more fields selected. They are all joined correctly (for lookup values etc.).

The concatenation was determined to be the bottleneck in performance. ol was defined as the import key to be used and the users were just entering the order number plus a 1 to get ONLY the first line number. Each order (usually) has more than one line and even the line number is not necessarily unique for each order. For instance, order 123 may have line 1 twice and line 2 once.

select order_number, line_number, address_field1 from order where rownum = 1;

The software appended the " and order_number = 123" as expected but because rownum = 1 had already been executed (before the and order_number = 123" the view only returned one row even WITHOUT the order_number = 123.

I need to return only ONE row for a given order number EVEN IF there are multiple records with the same order_number and line_number. Using "where order_number = <enteredOrderNumber> and line_number = 1" is not necessarily unique so that won't work.If I can't make it work this way and can't use the rownum = 1 what options do I have?