把不同结果集合并

--把不同结果集合并
select 
--a.docno adocno,
--b.docno bdocno,
CASE WHEN a.docNo IS NULL THEN b.docNo ELSE a.docNo END docNo,
CASE WHEN a.msg IS NULL  THEN '' ELSE a.msg END msg1,
CASE WHEN b.msg IS NULL  THEN '' ELSE b.msg END msg2
from 
(
    select docNo,'当前单据已被入库单调用，不允许回退！' as msg
    from dbo.ACN_PrePay
    where selfStatus = '生效' 
    and docno = 'YF1206110001' 
    and docno in 
    (
        select preReceiveDocNo from dbo.STG_ImpOrderWriteoff
        where preReceiveDocNo <> ''
    )
) a
full join
(
    select 
    docNo,'当前单据已被付款单调用，不允许回退！' as msg
    from dbo.ACN_PrePay
    where selfStatus = '生效' 
    and docno = 'YF1206110001' 
    and docno in 
    (
        select bizDocNo from dbo.ACN_PayPrePay
        where bizType = '预付货款单'
        and bizDocNo <> ''
    )
) b
on a.docNo=b.docNo