例如
select * from tbl_a a
left join tbl_b b on b.docno=a.docno and b.project=a.project
where a.docno='xxx'
发现a的project列和b的project列都为 null
于是匹配不到值
最后改为：
select * from tbl_a a
left join tbl_b b on b.docno=a.docno and isnull(b.project,0)=isnull(a.project,0)
where a.docno='xxx'
即可