StackOverflow 来拯救!我需要在一个查询调用中一次找到五列的中位数。

下面的中位数计算适用于单列，但组合使用时，多次使用“rownum”会导致查询失败。我如何更新它以适用于多列？谢谢你。这是为了创建一个网络工具，非营利组织可以在其中将他们的财务指标与用户定义的同行组进行比较。

SELECT t1_wages.totalwages_pctoftotexp AS median_totalwages_pctoftotexp
FROM (

SELECT @rownum := @rownum +1 AS  `row_number` , d_wages.totalwages_pctoftotexp
FROM data_990_c3 d_wages, (

SELECT @rownum :=0
)r_wages
WHERE totalwages_pctoftotexp >0
ORDER BY d_wages.totalwages_pctoftotexp
) AS t1_wages, (

SELECT COUNT( * ) AS total_rows
FROM data_990_c3 d_wages
WHERE totalwages_pctoftotexp >0
) AS t2_wages
WHERE 1 
AND t1_wages.row_number = FLOOR( total_rows /2 ) +1

--- [that was one median, below is another] ---

SELECT t1_solvent.solvent_days AS median_solvent_days
FROM (

SELECT @rownum := @rownum +1 AS  `row_number` , d_solvent.solvent_days
FROM data_990_c3 d_solvent, (

SELECT @rownum :=0
)r_solvent
WHERE solvent_days >0
ORDER BY d_solvent.solvent_days
) AS t1_solvent, (

SELECT COUNT( * ) AS total_rows
FROM data_990_c3 d_solvent
WHERE solvent_days >0
) AS t2_solvent
WHERE 1 
AND t1_solvent.row_number = FLOOR( total_rows /2 ) +1

[那是两个 - 总共有五个我最终需要立即找到中位数]

最佳答案

这种东西在MySQL中是个大痛。如果您要进行大量的统计排名工作，使用免费的 Oracle Express Edition 或 postgreSQL 可能是明智的。它们都有 MEDIAN(value) 聚合函数，这些函数是内置的或作为扩展提供的。这是一个小的 sqlfiddle 演示。 http://sqlfiddle.com/#!4/53de8/6/0

但你没有问过这个。

在 MySQL 中，您的基本问题是像@rownum 这样的变量的范围。您还有一个旋转问题:也就是说，您需要将查询的行转换为列。

让我们先解决主元问题。您要做的是创建几个大查询的联合。例如:

SELECT 'median_wages' AS tag, wages AS value
  FROM (big fat query making median wages) A
 UNION
SELECT 'median_volunteer_hours' AS tag, hours AS value
  FROM (big fat query making median volunteer hours) B
 UNION
SELECT 'median_solvent_days' AS tag, days AS value
  FROM (big fat query making median solvency days) C

下面是标签/值对表中的结果。您可以像这样旋转该表，以获得每一列中都有一个值的行。

SELECT SUM( CASE tag WHEN 'median_wages' THEN value ELSE 0 END 
          ) AS median_wages, 
SELECT SUM( CASE tag WHEN 'median_volunteer_hours' THEN value ELSE 0 END
          ) AS median_volunteer_hours, 
SELECT SUM( CASE tag WHEN 'median_solvent_days' THEN value ELSE 0 END 
          ) AS median_solvent_days
FROM (
    /* the above gigantic UNION query */
 ) Q

这就是将行(在这种情况下从 UNION 查询)向上旋转到列的方式。这是有关该主题的教程。 http://www.artfulsoftware.com/infotree/qrytip.php?id=523

现在我们需要处理中值计算子查询。您问题中的代码看起来不错。我没有您的数据，所以我很难对其进行评估。

但是您需要避免重复使用@rownum 变量。在您的一个查询中称它为@rownum1，在下一个查询中称它为@rownum2，依此类推。这是一个极小的 sql fiddle ，只执行其中一个。 http://sqlfiddle.com/#!2/2f770/1/0

现在让我们稍微构建一下，使用两个不同的中位数。这是 fiddle http://sqlfiddle.com/#!2/2f770/2/0这是 UNION 查询。 注意联合查询的后半部分使用@rownum2 而不是@rownum。

最后，这是带有旋转的完整查询。 http://sqlfiddle.com/#!2/2f770/13/0

 SELECT SUM( CASE tag WHEN 'Boston' THEN value ELSE 0 END ) AS Boston,
           SUM( CASE tag WHEN 'Bronx' THEN value ELSE 0 END ) AS Bronx   
   FROM (
 SELECT 'Boston' AS tag, pop AS VALUE
  FROM (
        SELECT @rownum := @rownum +1 AS  `row_number` , pop
          FROM pops, 
        (SELECT @rownum :=0)r
          WHERE pop >0 AND city = 'Boston'
          ORDER BY pop
        ) AS ordered_rows, 
        ( 
         SELECT COUNT( * ) AS total_rows
           FROM pops
          WHERE pop >0 AND city = 'Boston'
        ) AS rowcount
  WHERE ordered_rows.row_number = FLOOR( total_rows /2 ) +1
  UNION ALL
 SELECT 'Bronx' AS tag, pop AS VALUE
  FROM (
        SELECT @rownum2 := @rownum2 +1 AS  `row_number` , pop
          FROM pops, 
        (SELECT @rownum2 :=0)r
          WHERE pop >0 AND city = 'Bronx'
          ORDER BY pop
        ) AS ordered_rows, 
        ( 
         SELECT COUNT( * ) AS total_rows
           FROM pops
          WHERE pop >0 AND city = 'Bronx'
        ) AS rowcount
  WHERE ordered_rows.row_number = FLOOR( total_rows /2 ) +1
) D

这只是两个中位数。你需要五个。我认为很容易证明这种中值计算在 MySQL 中在单个查询中执行起来异常困难。

关于mysql - 在一次查询调用中计算同一张表中多列的中位数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/17447917/