如何在 CASE WHEN 中有多个 THEN 查询？答案

【问题标题】：How to have a CASE WHEN query with multiple THEN in it?如何在 CASE WHEN 中有多个 THEN 查询？
【发布时间】：2017-02-27 12:26:48
【问题描述】：

我的代码写成：

SELECT name, age,
  CASE WHEN abs(10 - mean(valueLeft)) < abs(10 - mean(valueRight)) THEN stddev(valueLeftMagnitude) ELSE stddev(valueRightMagnitude) END as value1,
  CASE WHEN abs(10 - mean(valueLeft)) < abs(10 - mean(valueRight)) THEN mean(valueLeftMagnitude) ELSE mean(valueRightMagnitude) END as value2,
  CASE WHEN abs(10 - mean(valueLeft)) < abs(10 - mean(valueRight)) THEN median(valueLeftMagnitude) ELSE median(valueRightMagnitude) END as value3
FROM table
GROUP BY name, age

由于所有三个 CASE 中的 WHEN 语句都是相同的，我不想重复它们。有什么办法吗？

示例：我想要类似 -

SELECT name, age,
  CASE WHEN abs(10 - mean(valueLeft)) < abs(10 - mean(valueRight))
    THEN stddev(valueLeftMagnitude) ELSE stddev(valueRightMagnitude) END as value1,
    THEN mean(valueLeftMagnitude) ELSE mean(valueRightMagnitude) END as value2,
    THEN median(valueLeftMagnitude) ELSE median(valueRightMagnitude) END as value3
FROM table
GROUP BY name, age

请帮忙。

【问题讨论】：

没有。剪切&粘贴&修改。
标记您的 DBMS。 sql-server 有交叉应用，在这里可以很方便。
嵌入式选择也很有用，即嵌入式查询进行计算，外部查询进行选择。

标签： sql pyspark

【解决方案1】：

这就是嵌入式选择如何确保您只执行一次case 计算：

SELECT perprocessed.name, perprocessed.age, stddev(perprocessed.valueMagnitude), mean(perprocessed.valueMagnitude), median(perprocessed.valueLeftMagnitude) from
(SELECT
  name,
  age,
  CASE WHEN abs(10 - mean(valueLeft)) < abs(10 - mean(valueRight)) THEN valueLeftMagnitude ELSE valueRightMagnitude END as valueMagnitude
  FROM table) as perprocessed
GROUP BY perprocessed.name, perprocessed.age

【讨论】：

【解决方案2】：

您的查询非常易读，我认为复制粘贴表达式没有真正的缺点。如果您只想进行一次此评估，则查询会稍微复杂一些，因为您需要分两步进行：

SELECT
  CASE WHEN which = 'left' THEN stddev_left ELSE stddev_right END AS value1,
  CASE WHEN which = 'left' THEN mean_left   ELSE mean_right   END AS value2,
  CASE WHEN which = 'left' THEN median_left ELSE median_right END AS value3
FROM
(
  SELECT 
    name, 
    age, 
    CASE WHEN abs(10 - mean(valueLeft)) < abs(10 - mean(valueRight)) 
         THEN 'left' ELSE 'right' END AS which,
    stddev(valueLeftMagnitude) AS stddev_left,
    stddev(valueRightMagnitude) AS stddev_right,
    mean(valueLeftMagnitude) AS mean_left,
    mean(valueRightMagnitude) AS mean_right,
    median(valueLeftMagnitude) AS median_left,
    median(valueRightMagnitude) median_right
  FROM table 
  GROUP BY name, age
) grouped;

【讨论】：