SQL stands for "Structured Query Language", and it is a fourth-generation computer language for defining, manipulating, and querying data from and in a Relational Database Management System (RDBMS).
The number of people who pronounce SQL as "SEQUEL" rather than "S-Q-L" (ess-queue-ell) increases as time goes on, and I've found the majority of those in the Sequel camp to be so for one of the following reasons:
* Microsoft officially supports the pronunciation "Sequel Server" for its SQL Server product
* It's shorter and easier to say
Those in the S-Q-L camp tend to cite one of the following:
* IBM had a SEQUEL language (Structured English QUEry Language) which predated SQL, thus causing ambiguity when Sequel is used to mean SQL
* The ANSI standard for SQL notes the pronunciation to be S-Q-L
My own findings and opinions on the topic:
* Wikipedia lists S-Q-L as the correct pronunciation with Sequel as an acceptable alternate pronunciation
* The historical tidbit with IBM reminds me too much of the GIF vs. JIFF thing--JIFF was an image format that never took off. If you pronounce GIF like JIFF, then it could cause ambiguity and so on. Sound familiar? It's noble, even cute, but at the end of the day, too pedantic; that line of argument doesn't fly with me.
* All three letter acronyms (TLAs) are spelled out. Seriously, try to think of one that isn't. CIA. FBI. NBA. NFL. CNN. NBC. BBC. AOL. MIA. AFK. You could pronounce some of them, e.g., Missing In Action (MIA), as though it rhymes with "via", but you'd look like an idiot. It's only once you hit four or five letters that this becomes cumbersome and you start finding creative ways to pronounce them instead. SQL is a TLA. Thus, you spell it out. QED.