Is it possible to either update or insert in a single query?

[Douglas Pearson]

Apologies if this is a dumb question, but is it possible to write a single
query that either updates certain columns in a row, or adds an entirely new
row if there is none already?

I seem to be running into this a lot, and so far I've solved it by:
1) run UPDATE table SET x,y WHERE some row
2) if rowsChanged == 0 then run the INSERT 

It just feels like there must be a way to do this more efficiently.

Thanks,

Doug