SQL is a language that MySQL uses. Read about SQL standards here :
http://epoch.cs.berkeley.edu:8000/sequoia/dba/montage/FAQ/SQL_TOC.html
http://www.jcc.com/SQLPages/jccs_sql.htm
Read about MySQL here :
http://mysql.com/information/
And through all that it should make sense :-) Fact is, MySQL does not support all of SQL, has some of its own functions, etc. But it supports most of it. Think of SQL as a language, MySQL communicates with and through it but doesn't have the entire vocabulary and has added a few words itself. The degree of this deviation depends on each database has its differences, this is why so many SQL databases exist. Check out :
http://www.codehound.com/SQL/
It really is nice to find such differences.