a join b 结果是交集
a left join b 结果的条数是a的条数
full out join 并集
具体见：
http://cms.csdnimg.cn/article/201409/29/5428ad03c3fd6.jpg
mysql常用命令
http://www.chinaz.com/program/2009/0115/62526.shtml
mysql -uroot -paaaaaa -h192.168.0.110
grep命令
http://www.cnblogs.com/ggjucheng/archive/2013/01/13/2856896.html
mysql简单练习题
http://blog.sina.com.cn/s/blog_767d65530101861c.html