sql left join 的探讨

Question

select a.xx,b.xx,c.xx from table1 a
left join table2 b on a.xxx = b.xxx
left join table3 c on a.xxx = c.xxx

select a.xx,b.xx,c.xx from table1 a
left join table2 b on a.xxx = b.xxx
left join table3 c on b.xxx = c.xxx

select ab.* from (select a.xx,b.xx from table1 a
left join talbe2 b on a.xxx = b.xxx) ab
left join table3 c on ab.xxx = c.xxx

假设没有语法错误，请问这3种情况查出的数据会是一样的嘛？
我自己test了一下，数据是一样的，但是我感觉跟我理解的left join不一样，不知道是不是我的理解压根就是有问题的
还请大神赐教，以纠正或者肯定我的理解，谢谢

Answer 1

1 式和 2 式是不一样的
3 式视取哪个表的 xxx 而定， a 表的就和 1 式一样，反之与 2 时一样
看个测试：

 create temporary table A
 select 1 as a
 union select 2
 union select 3;

create temporary table B
 select 1 as b
 union select 2;

create temporary table C
 select 1 as c
 union select 2
 union select 3;

select * from A
 left join B on a.a=b.b
 left join C on a.a=c.c;

select * from A
 left join B on a.a=b.b
 left join C on b.b=c.c;

select * from
 (select * from A left join B on a.a=b.b) ab
 left join C on ab.b=c.c;