表A:
a1 a2
1 2
3 8表B:
b1 b2
3 7
5 2
表C:
c1 c2
1 7
8 2
select * from a left join b on a.a1 =b.b1 请问这个结果是多少?
select * from a left join b on a.a1 =b.b1 and b.b1 = 3 请问这个结果是多少?
select * from a
left join b on a.a1 =b.b1
left join c on a.a2 = c.c1
请问这个结果是多少?
a1 a2
1 2
3 8表B:
b1 b2
3 7
5 2
表C:
c1 c2
1 7
8 2
select * from a left join b on a.a1 =b.b1 请问这个结果是多少?
select * from a left join b on a.a1 =b.b1 and b.b1 = 3 请问这个结果是多少?
select * from a
left join b on a.a1 =b.b1
left join c on a.a2 = c.c1
请问这个结果是多少?
楼主的前面2条select 返回的结果肯定是一样的
但是如果把*改成b.*结果就不一样了我同时还比较了另外的一种情况
select * from a left join b on a.a1 =b.b1 and b.b1 = 3
与
select * from a left join b on a.a1 =b.b1 where b.b1 = 3
楼主也可以一起试一试
与
select * from a left join b on a.a1 =b.b1 where b.b1 = 3
这两个结果不一样么?