Structured Query Language (SQL) is an ANSI (American National Standards Institute) Standard computer language. It is used to manage data stored in databases. Its basic functions are inputting, modifying, and deleting data from databases.
Although SQL is an ANSI standard, there are many flavors of the SQL language availble. For each of these variations to be compliant with the ANSI standard, they must all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar way.
Here are a a few tutorials that cover specific topics that you may find of interest: