What is SQL?

SQL or structured query language is a 4th generation language (4GL) where you just have to mention what you want to do, and not how do it. It is used to create, manipulate and retrieve information from RDBMS (Relational Database Management Systems).

Some common relational database management systems that use SQL are: Oracle, Sybase, Microsoft SQL Server, Access, Ingres, etc. Although most database systems use SQL, most of them also have their own additional proprietary extensions that are usually only used on their systems.

For history of SQL you can have a look at Wiki which says:

After testing SQL at customer test sites to determine the usefulness and practicality of the system, IBM began developing commercial products based on their System R prototype including System/38, SQL/DS, and DB2, which were commercially available in 1979, 1981, and 1983, respectively.

SQL was initially developed at IBM by Donald D. Chamberlin and Raymond F. Boyce in the early 1970s. This version, initially called SEQUEL (Structured English Query Language), was designed to manipulate and retrieve data stored in IBM's original quasi-relational database management system, System R, which a group at IBM San Jose Research Laboratory had developed during the 1970s. The acronym SEQUEL was later changed to SQL because "SEQUEL" was a trademark of the UK-based Hawker Siddeley aircraft company.

In the late 1970s, Relational Software, Inc. (now Oracle Corporation) saw the potential of the concepts described by Codd, Chamberlin, and Boyce and developed their own SQL-based RDBMS with aspirations of selling it to the U.S. Navy, Central Intelligence Agency, and other U.S. government agencies. In June 1979, Relational Software, Inc. introduced the first commercially available implementation of SQL, Oracle V2 (Version2) for VAX computers.

SQL or "sequel" ?

The above article from wiki clears all our doubts for the pronunciation part of it. But still.. you don't have to worry about it, the pronunciation would come automatically. When I entered the field of databases, I used to say "S-Q-L" for where ever I saw SQL. Now its like "MySQL" , "Sequel Server" or "PL/sequel". The more you love the technology the more you speak of it, the more you speak of it the more you are drawn towards the ease of saying it. SQL has three syllable sequel has two, so good for the lazy developers.