mysql:
http://www.mysqlops.com/2011/10/26/mysql-variable-third.html
http://linuxguest.blog.51cto.com/195664/718576
c:
http://www.baike.com/wiki/%E8%BE%97%E8%BD%AC%E7%9B%B8%E9%99%A4%E6%B3%95
