有没有办法在 UNION 之后得到第一行？

Question

我正在尝试获取两列中重复次数最多的值，此查询运行正常，我得到了预期的结果。但我只想要第一行，可以用 ROWNUM =1 得到吗？或者类似的东西？

我的环境是 Oracle 11 和 SQL 开发人员。

我的代码：

SELECT AIR_ARB, COUNT(*) FROM
(
SELECT AIR_ARB FROM AIR_INFO  WHERE  V_VX IN (910208,910209,9......) AND V_Y IN
(826369,826370,8.....)
UNION ALL
SELECT AIR_MAT FROM AIR_INFO WHERE  V_VX IN (910208,910209,9......) AND V_Y IN 
(826369,826370,8.....)

) t
GROUP BY AIR_ARB
ORDER BY COUNT (*) desc

我的结果：

AIR_ARB COUNT(*)
null     64
100      23
0        20
72       15
143       3
43        3

Answer 1

您需要一个额外的子查询：

SELECT a.*
FROM (SELECT AIR_ARB, COUNT(*) as cnt
      FROM (SELECT AIR_ARB FROM AIR_INFO  WHERE  V_VX IN (910208,910209,9......) AND V_Y IN (826369,826370,8.....)
            UNION ALL
            SELECT AIR_MAT FROM AIR_INFO WHERE  V_VX IN (910208,910209,9......) AND V_Y IN(826369,826370,8.....)
           ) t
      GROUP BY AIR_ARB
      ORDER BY COUNT (*) desc
     ) a
WHERE rownum = 1;

Answer 2

您还可以使用百分比来限制结果，如下所示，在 "ORDER BY"

之后添加以下表达式

 FETCH FIRST 0.01 PERCENT ROWS ONLY

Answer 3

如果您的 IN 列表相同，那么您可以使用 UNPIVOT 然后 GROUP BY 和 ORDER BY 然后在排序后按 WHERE ROWNUM = 1 过滤。

这意味着您只需要编写一次列表（使查询更易于调试和维护）并且 Oracle 只需要扫描 AIR_INFO table/index 一次（而 UNION 可以使用两次扫描）。

SELECT *
FROM   (
  SELECT value AS air_arb,
         COUNT(*)
  FROM   AIR_INFO
  UNPIVOT ( value FOR type IN ( AIR_ARB, AIR_MAT ) )
  WHERE  V_VX IN (910208,910209)
  AND    V_Y  IN (826369,826370)
  GROUP BY value
  ORDER BY COUNT(*) DESC
)
WHERE rownum = 1;

所以对于测试数据：

CREATE TABLE air_info ( v_vx, v_y, air_arb, air_mat ) AS
SELECT 910208, 826369, 1, 2 FROM DUAL UNION ALL
SELECT 910209, 826370, 3, 2 FROM DUAL;

这输出：

AIR_ARB | COUNT(*)
------: | -------:
      2 |        2

db<>fiddle here

有没有办法在 UNION 之后得到第一行？

There is a way to get the first row after UNION?

sql

oracle

oracle11g

rownum