SQL is an initialism for Structured Query Language. So the correct pronunciation is "s-q-l". Acronyms have a natural flow and can be read as is (NASA, LASER etc.). Initialisms do not and must be read one letter at a time.
Few language rules are absolute and universal. Language is, at its heart, an agreement among people about what words mean what. So the fact that many, many people say (and understand) "sequel" means that it can also be correct.
Except we are talking about the English language. And it does have rules, they are called grammar. Plus, you're using the bandwagon fallacy. Just because a lot of people do it doesn't make it right.
I thought not, and that was why some dictionaries started adding words like "irregardless" (sic).
Maybe this is just a skewed point of view of a Spanish native speaker, as we have an official organism accepted by nearly all Spanish speaking countries, that looks after the language and updates the rules at least once per year.
Go read a copy of The Canterbury Tales in it's original english and tell if it follows those rules. Languages are not set in stone they evolve. Just because a lot of people do it is exactly what language "rules" are, nothing more.
First of all, "bandwagon fallacy" doesn't make sense here. If I said, "English as it's commonly used is correct because everyone tells me it is," you could say that I'm jumping on a bandwagon.
What I'm saying is that "English as it's commonly used is correct because that's what defines the language itself." Language is, by its definition, a function of bandwagoning.
Second, you're utterly wrong about there being hard and fast rules to language. It's important that you look this up, or you'll go through more of your life being upset about something trivial and subjective.
If you do a little googling, I'm sure you'll see that there is no central authority to define the rules of language. In fact, for any language, there are many competing standards.
The "rules" you seem to be referring to (which I'm not limiting to grammar, but also including pronunciation, syntax, etc.) are prescriptive. Someone codified them at some point, and they were adopted by a general population.
But there are also descriptive rules for language, which refer to how people actually speak and write. These are often incorporated into newly-published or newly-revised sets of prescriptive rules. Descriptive rules may also change due to prescriptions, as is the case of avoiding ending a sentence in a preposition.
So both prescriptive rules and descriptive rules are valid, and there's no single source of "truth" for any of them. Here are some examples of things that many people (you included?) probably think are forbidden in English but are actually fine:
4. From http://en.wikipedia.org/wiki/Split_infinitive: "As the split infinitive became more common in the 19th century, some grammatical authorities sought to introduce a prescriptive rule against it. The construction is still the subject of disagreement among English speakers as to whether it is grammatically correct or good style: "No other grammatical issue has so divided English speakers since the split infinitive was declared to be a solecism in the 19c [19th century]: raise the subject of English usage in any conversation today and it is sure to be mentioned. However, most modern English usage guides have dropped the objection to the split infinitive."
If I follow your logic properly, then, when reading the sentence
"The disk with all the PNG files was read too frequently for the IDE controller, so we switched to a SCSI drive"
you would pronounce "PNG" as "Pea En Gee", "IDE" as "Eyed", and "SCSI" as "Ess See Ess Eye"? I would have pronounced "PNG" as "Ping", "IDE" as "Eye Dee Ee", and "SCSI" as "Scuzzy". I wonder if our differences are regional?
For the record, I pronounce "SQL" the same was that you do.
I don't think the word "incorrect" means what you think it means.
> Because you just have to ask one question: who decides the vowels?
As with pretty much everything else in language, its decided by what people use and understand. Its not, of course, necessarily obvious on the face of a string of letters how it will get turned into an oral utterance, and, yes, some things written as initialism that end up treated as acronyms in speech do so by having vowel sounds inserted that are not unambiguously inferable from the written initialism.
> Why "sequel" instead of "squel" or "skull".
Because language is memes (transmitted, repeatable behaviors) and particular memes have been created and spread related to the terms at issue.
> Isn't it just something a guy said once and everybody else just accepted as correct?
"Sequel". But I also used to pronounce LED (light-emitting diode) as "lead" until I heard an optical engineer (with PhD) say "l-e-d" and felt stupid. "Ping"(PNG) still sounds weird to me but scuzzy(SCSI) and as-key(ASCII) don't. It's a mystery.
