SQL 三表连接

http://www.111cn.net/database/mysql/62555.htm

select * from a left join b on a.id=b.id left join c on b.id=c.id and ***

注意是用 join ... on ... and ...，还是用 join ... on ... where ...

（现在看来，这个用的真是太普遍了，且何止三个。)