首先请看一语句：
select * from bj_productsys as a left join bj_orderformsys as b on a.productsys2=b.orderformsys4 where b.orderformsys25=199 and TO_DAYS(NOW()) - TO_DAYS(b.orderformsys11) <=0 order by a.productsys2 desca表肯有很多记录 where后的条件只涉及到b表，b表满足条件的就只有两条数据，结果我查询出来就就只两天数据，
我把left 改成inner join 结果，查询结果一模一样，改成right join 查询到的结果是对的。
请高手解答一下，这是什么情况。
谢谢！！
select * from bj_productsys as a left join bj_orderformsys as b on a.productsys2=b.orderformsys4 where b.orderformsys25=199 and TO_DAYS(NOW()) - TO_DAYS(b.orderformsys11) <=0 order by a.productsys2 desca表肯有很多记录 where后的条件只涉及到b表，b表满足条件的就只有两条数据，结果我查询出来就就只两天数据，
我把left 改成inner join 结果，查询结果一模一样，改成right join 查询到的结果是对的。
请高手解答一下，这是什么情况。
谢谢！！
select *
from bj_productsys as a left join
(select * from bj_orderformsys where orderformsys25=199 and TO_DAYS(NOW()) - TO_DAYS(orderformsys11) <=0 ) as b
on a.productsys2=b.orderformsys4
order by a.productsys2 desc