There is an official SQL standard from the ISO/ANSI committee.
Some information can be found here: http://en.wikipedia.org/wiki/Sql
However the specification itself is not freely available. You might find some drafts of older specs in the internet if you search for it.
A nice explanation of the (yet a bit old) ANSI 99 standard is here: https://mariadb.com/kb/en/sql-99-complete-really/
Here is a compilation of at least the BNF syntax for various versions of the standard