hi
i tried to do this ..
hope this helps 
please click arrow to the left for drop create data script
----------------------
-- create table
create table data
(
buy_currency varchar(10) , sell_currency varchar(10)
)
go
---------------------------
-- insert into data
insert into data select 'USD','ZAR'
insert into data select 'PAK','ARG'
insert into data select 'SAD','CIA'
insert into data select 'LAN','PUK'
go
select case when buy_currency > sell_currency then sell_currency else buy_currency end as currency1
, case when buy_currency > sell_currency then buy_currency else sell_currency end as currency2
from data
