Question

获取结果（rownum = 1）：

SELECT rownum,
       A.loadplan_name
FROM   (SELECT loadplan_name,
               run_date
        FROM   dcbp13_bia_odirepo.citizen_odi_required_loadplans
        WHERE  run_date LIKE '13-FEB-19'
        ORDER  BY run_date DESC,
                  loadplan_name ASC) A
WHERE  rownum = 1

未得到结果（rownum ≠ 1）：

SELECT rownum,
       A.loadplan_name
FROM   (SELECT loadplan_name,
               run_date
        FROM   dcbp13_bia_odirepo.citizen_odi_required_loadplans
        WHERE  run_date LIKE '13-FEB-19'
        ORDER  BY run_date DESC,
                  loadplan_name ASC) A
WHERE  rownum = 2

Answer 1

这在documentation中有解释。返回结果集时计算rownum。因此，它仅在将新行放入结果集中时才递增。

因此，在没有值“ 1”的情况下，它永远不会取值“ 2”。

如所解释的：

对ROWNUM值大于正整数的条件进行测试总是假的。例如，此查询不返回任何行：
SELECT *
FROM employees
WHERE ROWNUM > 1;
提取的第一行的ROWNUM分配为1，条件错误。现在要获取的第二行是第一行，还为ROWNUM分配了1，并使条件为假。所有行随后无法满足该条件，因此不会返回任何行。

解决方案通常是这样的：

SELECT rn, A.LOADPLAN_NAME
FROM (SELECT LOADPLAN_NAME, RUN_DATE, rownum as rn
      FROM DCBP13_BIA_ODIREPO.CITIZEN_ODI_REQUIRED_LOADPLANS      
      WHERE RUN_DATE like '13-FEB-19'
      ORDER BY RUN_DATE DESC, LOADPLAN_NAME ASC
     ) A  
WHERE rn = 2;

Answer 2

您非常亲密。正如doco所说，对大于正整数的ROWNUM值的运算始终为false。您可以使用小于，等于而不是大于。因此，此查询可以：

SELECT rownum,
       A.loadplan_name
FROM   (SELECT loadplan_name,
               run_date
        FROM   dcbp13_bia_odirepo.citizen_odi_required_loadplans
        WHERE  run_date LIKE '13-FEB-19'
        ORDER  BY run_date DESC,
                  loadplan_name ASC) A
WHERE  rownum < 2;

SELECT rownum,
       A.loadplan_name
FROM   (SELECT loadplan_name,
               run_date
        FROM   dcbp13_bia_odirepo.citizen_odi_required_loadplans
        WHERE  run_date LIKE '13-FEB-19'
        ORDER  BY run_date DESC,
                  loadplan_name ASC) A
WHERE  rownum = 2;

但不是：

SELECT rownum,
       A.loadplan_name
FROM   (SELECT loadplan_name,
               run_date
        FROM   dcbp13_bia_odirepo.citizen_odi_required_loadplans
        WHERE  run_date LIKE '13-FEB-19'
        ORDER  BY run_date DESC,
                  loadplan_name ASC) A
WHERE  rownum > 1;

在Oracle SQL中获取rownum = 1而不是1以外的数据

2 个答案: