SELECT  *
FROM    ( SELECT    * ,
                    ( SELECT TOP(1)   xmwyh  --如果子查询结果集只返回一行,则没问题.否则失败.加个TOP(1)，此SQL正确
                      FROM      xt_xmryk
                      WHERE     rybh = a.bh
                    ) AS xmwyh ,
                    ( SELECT TOP(1)   xm --同上
                      FROM      xt_ryb
                      WHERE     bh = a.bh
                    ) AS xm ,
                    ( SELECT TOP(1)   lxmc --同上
                      FROM      ry_zjlxb
                      WHERE     zjlxid = a.zjlxid
                    ) AS lxmc
          FROM      ry_zjfjb a
        ) AS b

------解决方案--------------------
(select xmwyh from xt_xmryk where rybh=a.bh) as xmwyh ,
(select xm from xt_ryb where bh=a.bh) as xm ,
(select lxmc from ry_zjlxb where zjlxid=a.zjlxid)
上面这些子查询返回的值不唯一，就会这样，改成下面试试

SQL code
select a.*,b.xmwyh,c.xm,d.lxmc 
from ry_zjfjb a 
left join xt_xmryk b on a.bh=b.ybh
left join xt_ryb   c on a.bh=c.bh
left join ry_zjlxb d on a.zjlxid=d.zjlxid

------解决方案--------------------
探讨

SQL code

SELECT  *
FROM    ( SELECT    * ,
                    ( SELECT TOP(1)   xmwyh  --如果子查询结果集只返回一行,则没问题.否则失败.加个TOP(1)，此SQL正确
                      FROM      xt_xmryk
                      ……