The official correct pronunciation is to pronounce it letter by letter, but it is more commonly pronounced as sequel.

The following is a reprint:

Many English-speaking database professionals pronounce SQL as sequel, but the correct pronunciation should be S-Q-L ("ess kyoo ell"). You can Guessing the reasons for this mispronunciation, I think there are both historical and linguistic reasons.

For historical reasons, IBM developed a language called SEQUEL in the 1970s, which is the acronym for Structured English Query Language. This language was designed to manipulate data stored in the database system Sytem. Data in R, Sytem R is based on the relational database management system (RDBMS) model proposed by Dr. Edgar F. Codd. Later, due to a trademark dispute, the initials SEQUEL were simplified to SQL. ANSI selected SQL as a standard in 1986, and ISO did the same in 1987. ANSI announced that the correct pronunciation of the language is "ess kyoo ell," but it doesn't seem to be widely accepted.

Linguistically, sequel speaks more fluently, especially for English speakers. I myself use this pronunciation for this reason.

Sometimes you can guess which pronunciation is used by examining how people write it. Someone who writes "an SQL Server" may use the correct pronunciation, while someone who writes "a SQL Server" may use Wrong pronunciation.

p.s: It is also very interesting to learn about history

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