I spent some time studying this old problem, because it arose again in my company. As John Woo said, this is a matter of pronunciation, not spelling. SQL is pronounced "continued" by many, and there is some history, except for a simple attempt to make an abbreviation from initialism. (NASA is an abbreviation; IBM is initialism).
But in writing, the best and most consistent way to access SQL Server or SQL databases, etc. is "SQL" --which, in which the article "an" is based on the initial letter "S". Both IBM and Microsoft style guides agree on "SQL" in writing, even if it may be declared a "continuation" in some contexts.
Our company has been embarrassed by this issue for several years because a Google search shows a much wider use of "... SQL" in English hits. We also relied heavily on Microsoft's Style Guide, and their "rule" was easily misinterpreted. They updated it for the 4th edition - see pages 387-388. SO: In the conversation, do what you want, if you understand that the “continuation” works in English chat, but not necessarily for people whose mother tongue is not English. In writing, this is SQL, and the article is "an".
Sukoch
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