I was deep into this issue several years back, and I wish I still had my old notes so that I tell you which of the then-major RDMBSs didn't provide a count at SELECT time. It had to do with how that particular RDMBS used lazy evaluation to produce its result set. (Lazy eval meant that no count was avaible until the result set was exhausted. I doubt that's what you want.) This might not be an issue with the current generation of significant databases. Perhaps others who work with the major vendors can chime in and let us know.

If you've got something working with MySQL, a strategy of sticking with what you've got, but noting that there might be an issue should you port, may be good enough.