sql server关于left join的记录

现有approve和projectReg两张表，没有物理外键。

approve: businessCode,  type

projectReg: projectId

select * 
from projectReg a LEFT JOIN approve b  
on a.projectRegId = b.businessCode

select *
from approve a LEFT JOIN projectReg b
on a.businessCode=b.projectRegId

都会报错：[Err] 22018 - [SQL Server]在将 varchar 值 '3ad045bd10c1482d962ec944bca21056' 转换成数据类型 int 时失败。

select * 
from projectReg a LEFT JOIN approve b  
on a.projectRegId = b.businessCode and b.type=5

不报错

select * 
from projectReg a LEFT JOIN approve b  
on a.projectRegId = b.businessCode 
-- and b.type=5
where b.type=5

将过滤被连接的表的条件放到where中，同样不报错

select *
from approve a LEFT JOIN projectReg b
on a.businessCode=b.projectRegId and a.type=5

报错：[Err] 22018 - [SQL Server]在将 varchar 值 '3ad045bd10c1482d962ec944bca21056' 转换成数据类型 int 时失败。

select *
from approve a LEFT JOIN projectReg b
on a.businessCode=b.projectRegId 
-- and a.type=5
where a.type=5

这样也不报错

更有意思的是：

select *
from approve a LEFT JOIN projectReg b
on a.businessCode=b.projectRegId 
and a.type=5
where a.type=5

居然也不报错，很无奈啊！！！！

总结：

仅仅尝试了sql server

1.网上关于on和where条件是先生成临时表再用where去过滤的情况，在sql server中是不成立的。

2.关于对主表的条件过滤，应该放到where中