That said... As a native English speaker, I can infer your pronunciation from which of the options of "a" and "an" you decide to use.
FWIW, "an" is appropriate for any word starting with a vowel sound (whether or not it actually starts with a vowel) and inappropriate for any word that doesn't. "An historic" drives me nuts, but I'm one of those jerks. (My mother was an English teacher - I never stood an chance!)
On some level, I think you can still apply the "starts with a vowel" rule for SQL, since in this case "S" is really standing in for "ess" (assuming you're not saying "sequel", which I've never gotten into the habit of doing).
It's even simpler. The "starts with a vowel" rule is about pronunciation. The spelling is entirely orthogonal. If you pronounce "SQL" as "Ess-cue-ell", it's "an". If you pronounce it as "sequel", it's "a".
Aside: the reason "an" is also sometimes used for word starting with an "h" is that the "h" in those cases is silent and the pronounciation therefore starts with a vowel (following the silent "h"). There are some style guides that recommend using "an" for words where the "h" isn't silent (or "a" for words where it is) but they're just being intentionally obtrusive.
Yes - exactly my point. It is interesting to learn, as another commented somewhere in this thread, that my brain just invokes the appropriate "sequel" vs. S.Q.L. just from whether or not someone includes the ever-so-important "n."
It's not automatic for me, which is why I found it annoying enough to ask. If a writer writes 'an SQL query', that doesn't bother me at all, but if a writer writes 'a SQL query', it takes me a while before I start hearing it in my head as 'a sequel query' -- until then, it just sounds wrong.
Worse is that some writers seem to mix it up, use both periodically.