http://blog.csdn.net/jintao_ma/article/details/51260458
https://zhidao.baidu.com/question/1304158100359936619.html
多个表查询的时候,这些不同的连接类型可以写到一块。例如:
select t1.c1,t2.cx,t3.cy
from tab1 t1
?????? inner join tab2 t2 on (t1.c1=t2.c2)
?????? inner join tab3 t3 on (t1.c1=t2.c3)
?????? left outer join tab4 on(t2.c2=t3.c3);
where t1.x >t3.y;
上面这个sql查询是多表连接的一个示范