insert/update

I seemed to remember being able to do this but I can't find the docs.

Can I run a sql query to insert new or update existing rows in one query?

Otherwise I have to run a select query to see if it's there and then
another one to update/insert.

What I'm trying to do is create a counter for each key, insert a value
of 1 or increment the value by 1 and then set another specific row
(where key = $key) to always increment by 1.

And the more I type, the more this sounds like the answer is going to be
part function, part trigger....  Maybe I should post to 'novice' for a
while!  ;)