右链接问题
select a.pk_freevalue, b.freevalueid
from bjh_subjamount_tmp b,
(select pk_freevalue, trim(valuecode) as valuecode, subsys
from bjh_freevalue
where freetype = '00') a
where a.valuecode(+) = b.freevalueid
-- and a.subsys (+)= b.subsys
and b.freevalueid is not null
and b.subsys = 'CW'
and b.period = '201105';
最近我在一段源码中看到这样一段SQL,我不理解 and a.subsys (+)= b.subsys
他加上这句代码有什么用?