在CASE WHEN语句中不能使用SELECT，因此需要重写它。
CASE WHEN只能返回标量值，因此需要重写它
另外，JOIN作为SQL标准已经存在了30年，所以是时候使用它了。

select A.REGION  ,
       E.APPLY_ID    ,
       E.JOB_ID      , 
       E.LOGIN_ID    ,
       J.TITLE      
  from EMPLOYEE_JOBS E  
  INNER JOIN EMPLOYEE_JOB_LIST  J ON E.JOB_ID = J.JOB_ID
  INNER JOIN EMPLOYEE_APP A  ON E.LOGIN_ID = A.LOGIN_ID
  INNER JOIN COUNTRY C ON A.COUNTRY_UID = C.COUNTRY_ID
  INNER JOIN LOV_MISC L ON L.LOV_CD  = E.STATUS
  INNER JOIN LOV_MISC LL ON NVL(A.SHIFT_TIME,5) = LL.LOV_CD 
  where 
     L.LOV_GRP ='EMP_HIRING'   
  AND   E.STATUS = '1'
  AND   LL.LOV_GRP ='EMP_SHIFT'
  AND   UPPER(A.gender) = decode(&V_GENDER,'BOTH',UPPER(A.gender), &V_GENDER )
  AND   case when &P_REGION IS NOT NULL  then A.REGION IN ( select column_value  from table (DOY_FN_STR_TO_TBL(&P_REGION)))
                          else A.REGION = A.REGION end ;

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select column_value  from table (DOY_FN_STR_TO_TBL(&P_REGION))

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是问题中唯一的子查询;正如错误所述，它必须返回多行。
您需要限制子查询，使其仅返回单行：

select column_value
from   table (DOY_FN_STR_TO_TBL(&P_REGION))
where  rownum = 1

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否则，重写查询以删除CASE表达式并使用OR：

select A.REGION  ,
       E.APPLY_ID    ,
       E.JOB_ID      , 
       E.LOGIN_ID    ,
       J.TITLE      
from   EMPLOYEE_JOBS E
       INNER JOIN EMPLOYEE_JOB_LIST J
       ON (E.JOB_ID = J.JOB_ID)
       INNER JOIN EMPLOYEE_APP A
       ON (E.LOGIN_ID = A.LOGIN_ID)
       INNER JOIN COUNTRY C
       ON (A.COUNTRY_UID = C.COUNTRY_ID)
       INNER JOIN LOV_MISC L
       ON (L.LOV_CD  = E.STATUS)
       INNER JOIN LOV_MISC LL
       ON (NVL(A.SHIFT_TIME,5) = LL.LOV_CD)
where  L.LOV_GRP ='EMP_HIRING'
AND    E.STATUS = '1'
AND    LL.LOV_GRP ='EMP_SHIFT'
AND    (  &V_GENDER = 'BOTH'
       OR UPPER(A.gender) = &V_GENDER )
AND    (  A.REGION IN (select column_value  from table (DOY_FN_STR_TO_TBL(&P_REGION)))
       OR &P_REGION IS NULL );

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