标量子查询中有ROWNUM=1怎么改？

碰到标量子查询中有ROWNUM=1怎么改？

select to_date(o.postdate,'yyyymmdd'), 
         (select cur.c_code from cur_tbl cur where cur.c_abbr=o.currencycode) as CusNo, 
         o.dramount, 
         round(o.dramount *
               (select s.exechangerate / 100
                  from tcsa.uccexchange s
                 where s.exchangecurrency =
                       (SELECT T.t_code
                          FROM tcsa.sap_code_mapping t
                         where t.typename = 'currency'
                           and T.sap_code = o.currencycode)
                   and TO_CHAR(s.issuancedate, 'yyyymmdd') = o.postdate
                   and rownum = 1),2) AS Debit_Sum_RMB, 
         o.cramount,
         round(o.cramount *
               (select s.exechangerate / 100
                  from tcsa.uccexchange s
                 where s.exchangecurrency =
                       (SELECT T.t_code
                          FROM tcsa.sap_code_mapping t
                         where t.typename = 'currency'
                           and T.sap_code = o.currencycode)
                   and TO_CHAR(s.issuancedate, 'yyyymmdd') = o.postdate
                   and rownum = 1),2) AS Credit_Sum_RMB
    from tcsa.mm_sap_voucher_detail_to o
    where o.postdate >= '20170101'
         and o.postdate < '20180101' ;

Plan hash value: 1961056669

-------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                     | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     | Inst   |IN-OUT|
-------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |                          |  1210K|   639M|       |   153K  (1)| 00:30:48 |        |      |
|*  1 |  TABLE ACCESS FULL         | CUR_TBL                  |     1 |    12 |       |     3   (0)| 00:00:01 |        |      |
|*  2 |  COUNT STOPKEY             |                          |       |       |       |            |          |        |      |
|*  3 |   FILTER                   |                          |       |       |       |            |          |        |      |
|*  4 |    VIEW                    |                          | 31006 |  2240K|       |   778   (1)| 00:00:10 |        |      |
|*  5 |     WINDOW SORT PUSHED RANK|                          | 31006 |  2815K|  6632K|   778   (1)| 00:00:10 |        |      |
|   6 |      REMOTE                | UCCEXCHANGE              | 31006 |  2815K|       |   111   (1)| 00:00:02 |  CLONE | R->S |
|   7 |    REMOTE                  | SAP_CODE_MAPPING         |     1 |   240 |       |     2   (0)| 00:00:01 |  CLONE | R->S |
|*  8 |  COUNT STOPKEY             |                          |       |       |       |            |          |        |      |
|*  9 |   FILTER                   |                          |       |       |       |            |          |        |      |
|* 10 |    VIEW                    |                          | 31006 |  2240K|       |   778   (1)| 00:00:10 |        |      |
|* 11 |     WINDOW SORT PUSHED RANK|                          | 31006 |  2815K|  6632K|   778   (1)| 00:00:10 |        |      |
|  12 |      REMOTE                | UCCEXCHANGE              | 31006 |  2815K|       |   111   (1)| 00:00:02 |  CLONE | R->S |
|  13 |    REMOTE                  | SAP_CODE_MAPPING         |     1 |   240 |       |     2   (0)| 00:00:01 |  CLONE | R->S |
|  14 |  REMOTE                    | MM_SAP_VOUCHER_DETAIL_TO |  1210K|   639M|       |   153K  (1)| 00:30:48 |  CLONE | R->S |
-------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("CUR"."C_ABBR"=SYS_OP_C2C(:B1))
   2 - filter(ROWNUM=1)
   3 - filter("EXCHANGECURRENCY"= (SELECT /*+ */ "T"."t_code" FROM  "A2" WHERE "T"."TYPENAME"='currency' AND 
              "T"."SAP_CODE"=:B1))
   4 - filter(TO_CHAR("ISSUANCEDATE",'yyyymmdd')=:B1 AND "RN"=1)
   5 - filter(ROW_NUMBER() OVER ( PARTITION BY "T_RMB"."BASECURRENCY","T_RMB"."EXCHANGECURRENCY",TO_DATE(TO_CHAR(INTERN
              AL_FUNCTION("T_RMB"."ISSUANCEDATE"),'yyyymmdd'),'yyyymmdd') ORDER BY "STATUS")<=1)
   8 - filter(ROWNUM=1)
   9 - filter("EXCHANGECURRENCY"= (SELECT /*+ */ "T"."t_code" FROM  "A2" WHERE "T"."TYPENAME"='currency' AND 
              "T"."SAP_CODE"=:B1))
  10 - filter(TO_CHAR("ISSUANCEDATE",'yyyymmdd')=:B1 AND "RN"=1)
  11 - filter(ROW_NUMBER() OVER ( PARTITION BY "T_RMB"."BASECURRENCY","T_RMB"."EXCHANGECURRENCY",TO_DATE(TO_CHAR(INTERN
              AL_FUNCTION("T_RMB"."ISSUANCEDATE"),'yyyymmdd'),'yyyymmdd') ORDER BY "STATUS")<=1)

看到这种不加排序的子句直接使用rownum=1的查询，本身就是对数据要求不严格。所以只要达到它的一个特性（最多返回一行）就可以，上面的标量语句就可以改为：

(select max(s.exechangerate) exechangerate,
                           s.exchangecurrency,
                           TO_CHAR(s.issuancedate, 'yyyymmdd') issuancedate1
                      from tcsa.uccexchange s
                     group by TO_CHAR(s.issuancedate, 'yyyymmdd'),s.exchangecurrency)

整体改写后：

select postdate, CusNo, dramount, Debit_Sum_RMB, cramount, Credit_Sum_RMB
  from (select to_date(o.postdate, 'yyyymmdd') postdate,
               cur.c_code as CusNo,
               o.dramount,
               round(o.dramount * (s.exechangerate1 / 100), 2) AS Debit_Sum_RMB,
               o.cramount,
               round(o.cramount * (s.exechangerate1 / 100), 2) AS Credit_Sum_RMB,
               s.exchangecurrency,
               t.t_code
          from tcsa.mm_sap_voucher_detail_to o
          left join cur_tbl cur
            on cur.c_abbr = o.currencycode
          left join (select max(exechangerate) exechangerate1,
                           exchangecurrency,
                           TO_CHAR(issuancedate, 'yyyymmdd') issuancedate1
                      from tcsa.uccexchange
                     group by exchangecurrency,
                              TO_CHAR(issuancedate, 'yyyymmdd')) s
            on s.issuancedate1 = o.postdate
          left join tcsa.sap_code_mapping t
            on t.typename = 'currency'
           and t.sap_code = o.currencycode
         where o.postdate >= '20170101'
           and o.postdate < '20180101') tab
 where t_code = exchangecurrency;