mysql - SQL 查询返回最常见、第二常见、第三常见的情况

Question

我可以使用数据透视表在 Excel 中执行此操作，但我想找出一种直接从单个 SQL 查询执行此操作的方法。假设我有一份水果及其新鲜度 list :

**Fruit   Freshness**
Banana  New
Banana  Ripe
Apple   Ripe
Orange  Old
Cherry  New
Orange  Ripe
Apple   Old
Banana  Old
Apple   New
Apple   New
Orange  Ripe
Banana  Old
Orange  New
Cherry  New
Cherry  Ripe

我想计算“新鲜度”出现的次数，然后将它们排名为最频繁、第二频繁，依此类推。结果看起来像这样:

**Fruit Most common 2nd most common 3rd most common**
Banana  New         Old             Ripe
Apple   Old         New             Ripe
Orange  Ripe        New             Old
Cherry  New         Ripe            NA

这可以在一个查询中实现吗？

Answer 1

您可以使用 GROUP_CONCAT()聚合函数:

SELECT
  Fruit, GROUP_CONCAT(Freshness ORDER BY cnt DESC) as Common
FROM (
  SELECT Fruit, Freshness, COUNT(*) cnt
  FROM
    fruits
  GROUP BY
    Fruit, Freshness
) s

将返回如下值:

Fruit  | Common
---------------------
Banana | New,Old,Ripe
Cherry | New,Ripe
...    | ...

但是如果你想将结果分成三列，你可以结合前面的查询并使用 SUBSTRING_INDEX()从逗号分隔值中提取第一个、第二个和第三个值:

SELECT
  Fruit,
  SUBSTRING_INDEX(Common, ',', 1) AS most,
  CASE WHEN CommonLIKE '%,%'
       THEN SUBSTRING_INDEX(SUBSTRING_INDEX(Common, ',', 2), ',', -1) END AS second_most,
  CASE WHEN CommonLIKE '%,%,%'
       THEN SUBSTRING_INDEX(SUBSTRING_INDEX(Common, ',', 3), ',', -1) END AS third_most
FROM (
  SELECT
    Fruit, GROUP_CONCAT(Freshness ORDER BY cnt DESC) as Common
  FROM (
    SELECT Fruit, Freshness, COUNT(*) cnt
    FROM
      fruits
    GROUP BY
      Fruit, Freshness
  ) s
  GROUP BY
    Fruit
  ) s