正如注解中提到的，使用一个总是提供15行的帮助表。下面的例子是关于emp/dept样本数据的，但是你也应该能够对你的数据集这样做。emp表有14行。
步骤1：生成15个空行

select level
  from dual
connect by
   level - 1< 15

步骤2：向原始查询添加行号。这将用于连接到帮助器表。把顺序改为你需要满足的“前15名”条件。

select ROW_NUMBER() OVER (PARTITION BY 1 ORDER BY e.ename),  
       e.ename, 
       e.job 
  from emp e

现在把它们放在一起：

with dummy_rows (rn) as
(
select level
  from dual
connect by
    level - 1< 15
)
,
my_emp_query(rn, ename, job) as
(
select ROW_NUMBER() OVER (PARTITION BY 1 ORDER BY e.ename),  
       e.ename, 
       e.job 
  from emp e 
)
select d.rn, e.ename, e.job 
  from 
  dummy_rows d
  left outer join my_emp_query e on d.rn = e.rn    ;

        RN ENAME      JOB      
---------- ---------- ---------
         1 ADAMS      CLERK    
         2 ALLEN      SALESMAN 
         3 BLAKE      MANAGER  
         4 CLARK      MANAGER  
         5 FORD       ANALYST  
         6 JAMES      CLERK    
         7 JONES      MANAGER  
         8 KING       PRESIDENT
         9 MARTIN     SALESMAN 
        10 MILLER     CLERK    
        11 SCOTT      ANALYST  
        12 SMITH      CLERK    
        13 TURNER     SALESMAN 
        14 WARD       SALESMAN 
        15                     

15 rows selected.

注意第15行是空的，因为emp表中没有匹配的记录。把15改成5，你只会得到5行。

我做到了！可能Koen Lostrie的答案比我做的更好，但我得到了我公司某个人的帮助，我认为它最符合我们的标准。代码如下：

SELECT *
    FROM (SELECT IDCODFATURA,
                 PAGINA,
                 TARIFA_RES_TE,
                 VALOR_ICMS_TE,
                 TARIFA_COM_ICMS_TE,
                 TARIFA_SEM_ICMS_TE,
                 VALOR_PISCOFINS_TE,
                 VALOR_COM_ICMS_TE,
                 VALOR_SEM_ICMS_TE,
                 VALOR_BASE_ICMS,
                 TARIFA_TUSD,
                 CASE
                     WHEN NVL (param_com_tributos.tipo_imp_imposto_fat, 0) = 0
                     THEN
                         base_fatura_itens.valor_com_icms
                     ELSE
                         base_fatura_itens.VALOR_SEM_PCICMS
                 END                                AS valor,
                 CASE
                     WHEN NVL (param_com_tributos.tipo_imp_imposto_fat, 0) = 0
                     THEN
                         base_fatura_itens.tarifa_com_icms
                     ELSE
                         base_fatura_itens.tarifa_res
                 END                                AS tarifa,
                 TARIFA_TE,
                 usuario,
                 GUID,
                 TIPO_DESCONTO_MICROGERACAO,
                 (SELECT VALOR_PIS
                    FROM base_fatura_web
                   WHERE     IDCODFATURA = :IDCODFATURA
                         AND PAGINA = :PAGINA
                         AND GUID = :GUID
                         AND usuario = :usuario)    AS VALOR_PIS,
                 (SELECT VALOR_COFINS
                    FROM base_fatura_web
                   WHERE     IDCODFATURA = :IDCODFATURA
                         AND PAGINA = :PAGINA
                         AND GUID = :GUID
                         AND usuario = :usuario)    AS VALOR_COFINS
            FROM base_fatura_itens, param_com_tributos
           WHERE     IDCODFATURA = :IDCODFATURA
                 AND PAGINA = :PAGINA
                 AND param_com_tributos.idparam = 1
                 AND base_fatura_itens.GUID = :GUID
                 AND base_fatura_itens.usuario = :usuario
                 AND base_fatura_itens.descricao <>
                     'ICMS Descontos tarifários'
                 AND ROWNUM <= 15
          UNION ALL
              SELECT NULL     AS IDCODFATURA,
                     NULL     AS PAGINA,
                     NULL     AS TARIFA_RES_TE,
                     NULL     AS VALOR_ICMS_TE,
                     NULL     AS TARIFA_COM_ICMS_TE,
                     NULL     AS TARIFA_SEM_ICMS_TE,
                     NULL     AS VALOR_PISCOFINS_TE,
                     NULL     AS VALOR_COM_ICMS_TE,
                     NULL     AS VALOR_SEM_ICMS_TE,
                     NULL     AS VALOR_BASE_ICMS,
                     NULL     AS TARIFA_TUSD,
                     NULL     AS valor,
                     NULL     AS tarifa,
                     NULL     AS TARIFA_TE,
                     NULL     AS usuario,
                     NULL     AS GUID,
                     NULL     AS TIPO_DESCONTO_MICROGERACAO,
                     NULL     AS VALOR_PIS,
                     NULL     AS VALOR_COFINS
                FROM DUAL
          CONNECT BY LEVEL <=
                     (  15
                      - (SELECT COUNT (*)
                           FROM base_fatura_itens, param_com_tributos
                          WHERE     IDCODFATURA = :IDCODFATURA
                                AND PAGINA = :PAGINA
                                AND param_com_tributos.idparam = 1
                                AND base_fatura_itens.GUID = :GUID
                                AND base_fatura_itens.usuario = :usuario
                                AND base_fatura_itens.descricao <>
                                    'ICMS Descontos tarifários')))
   WHERE ROWNUM <= 15
ORDER BY classificacao, sequencia