oracle sql: update if exists else insert

Possible Duplicate:
Oracle: how to UPSERT (update or insert into a table?)

Hi,

I have a table in which a record has to be modified if it already exists else a new record has to be inserted. Oracle sql doesnt accept IF EXISTS, otherwise I would have done an if - update - else - insert query. I've looked at MERGE but it only works for multiple tables. What do i do?