[sql] How to fix Ora-01427 single-row subquery returns more than one row in select?

When i execute the following query, i get the message like

"Ora-01427 single-row subquery returns more than one row"

SELECT E.I_EmpID AS EMPID,
       E.I_EMPCODE AS EMPCODE,
       E.I_EmpName AS EMPNAME,
       REPLACE(TO_CHAR(A.I_REQDATE, 'DD-Mon-YYYY'), ' ', '') AS FROMDATE,
       REPLACE(TO_CHAR(A.I_ENDDATE, 'DD-Mon-YYYY'), ' ', '') AS TODATE,
       TO_CHAR(NOD) AS NOD,
       DECODE(A.I_DURATION,
              'FD',
              'FullDay',
              'FN',
              'ForeNoon',
              'AN',
              'AfterNoon') AS DURATION,
       L.I_LeaveType AS LEAVETYPE,
       REPLACE(TO_CHAR((SELECT C.I_WORKDATE
                         FROM T_COMPENSATION C
                        WHERE C.I_COMPENSATEDDATE = A.I_REQDATE
                          AND C.I_EMPID = A.I_EMPID),
                       'DD-Mon-YYYY'),
               ' ',
               '') AS WORKDATE,
       A.I_REASON AS REASON,
       AP.I_REJECTREASON AS REJECTREASON
  FROM T_LEAVEAPPLY A
 INNER JOIN T_EMPLOYEE_MS E
    ON A.I_EMPID = E.I_EmpID
   AND UPPER(E.I_IsActive) = 'YES'
   AND A.I_STATUS = '1'
 INNER JOIN T_LeaveType_MS L
    ON A.I_LEAVETYPEID = L.I_LEAVETYPEID
  LEFT OUTER JOIN T_APPROVAL AP
    ON A.I_REQDATE = AP.I_REQDATE
   AND A.I_EMPID = AP.I_EMPID
   AND AP.I_APPROVALSTATUS = '1'
 WHERE E.I_EMPID <> '22'
 ORDER BY A.I_REQDATE DESC

when i execute this without ORDER BY A.I_REQDATE DESC it returns 100 rows...

(SELECT C.I_WORKDATE
         FROM T_COMPENSATION C
         WHERE C.I_COMPENSATEDDATE = A.I_REQDATE AND ROWNUM <= 1
         AND C.I_EMPID = A.I_EMPID)

The only subquery appears to be this - try adding a ROWNUM limit to the where to be sure:

(SELECT C.I_WORKDATE
         FROM T_COMPENSATION C
         WHERE C.I_COMPENSATEDDATE = A.I_REQDATE AND ROWNUM <= 1
         AND C.I_EMPID = A.I_EMPID)

You do need to investigate why this isn't unique, however - e.g. the employee might have had more than one C.I_COMPENSATEDDATE on the matched date.

For performance reasons, you should also see if the lookup subquery can be rearranged into an inner / left join, i.e.

 SELECT 
    ...
    REPLACE(TO_CHAR(C.I_WORKDATE, 'DD-Mon-YYYY'),
            ' ',
            '') AS WORKDATE,
    ...
 INNER JOIN T_EMPLOYEE_MS E
    ...
     LEFT OUTER JOIN T_COMPENSATION C
          ON C.I_COMPENSATEDDATE = A.I_REQDATE
          AND C.I_EMPID = A.I_EMPID
    ...