mysql left join ,right join ,inner join

表A lefttable,表B righttable

left join: 与right join 相类似，反之亦然。

1. select lefttable.*,righttable.* from lefttable left join righttable on lefttable.id = righttable.id and righttable.name = "xxx" where lefttable.name = "XXX";

2. select lefttable.*,righttable.* from lefttable left join righttable on lefttable.id = righttable.id and leftable.name = "xxx" where lefttable.name = "XXX";

3. select lefttable.*,righttable.* from lefttable left join righttable on lefttable.id = righttable.id and righttable.name = "xxx" where righttable.name = "XXX";

说明：

　　1. left join : 以 lefttable为驱动表，驱动righttable 表，即使righttable 表没有与之相匹配的，也会显示出来，只是用null代替而已。

　　2. and righttable.name : 这里的条件只对被驱动表有效。如果针对的是驱动表则可忽视它的存在。

　　3. where ： 针对的是联合查询得到的结果集。不满足条件就过滤掉。

inner join

1. select lefttable.*,righttable.* from lefttable inner join righttable on lefttable.id = righttable.id and righttable.name = "xxx" where lefttable.name = "XXX";

2. select lefttable.*,righttable.* from lefttable inner [b]join[/b] righttable on lefttable.id = righttable.id and leftable.name = "xxx" where lefttable.name = "XXX";

3. select lefttable.*,righttable.* from lefttable inner [b]join[/b] righttable on lefttable.id = righttable.id and righttable.name = "xxx" where righttable.name = "XXX";

说明：

　　标红的三个地方不像left join ,right join ,所启动的作用都与where 一样的，不满足条件就过滤掉。