左连接left join 的空数据

select
        T1.serialno,t1.dwmonth,t2.dwmonth,T1.balance,t2.balance as ReportItem
from
        receive_badloan T1 left join a_contract T2 on
        t1.dwmonth=t2.dwmonth and t2.dwmonth='2005/10/31' and
        t1.serialno = t2.serialno and
        t1.businesscurrency = t2.businesscurrency where operateorg like '1011%'.
上面这句sql的要求是：最终显示的余额(balance)来自t2，serialno,dwmonth,businesscurrency是t2的三个主键。看起来这样写好像没问题，但是查询结果如下：
serialno                   t1.dwmonth       t2.dwmonth       t1.balance     reportItem
111998060004        2005-09-30                                  303697.6       
111998060004        2005-10-31        2005-10-31        278697.6        278697.6
，t2.balance有为空的值。查看原因发现 t1.dwmonth=t2.dwmonth为多余的条件，因为现在的要求是查询dwmonth为2005/10/31的余额，如果t1.dwmonth=t2.dwmonth了，则2005-09-30去关联t2 中 t2.dwmonth='2005/10/31'的数据当然 没得关联所以，t2.balance为空。
正确sql应该为：
select
        T1.serialno,t1.dwmonth,t2.dwmonth,T1.balance,t2.balance as ReportItem
from
        receive_badloan T1 left join a_contract T2 on
        t2.dwmonth='2005/10/31' and
        t1.serialno = t2.serialno and
        t1.businesscurrency = t2.businesscurrency where operateorg like '1011%'
正确结果如下：
111998060004        2005-09-30        2005-10-31        303697.6        278697.6
111998060004        2005-10-31        2005-10-31        278697.6        278697.6

左连接并非主键全部要等值起来!