获取最大计数的行

Posted 2023-03-31

【中文标题】获取最大计数的行

【英文标题】：Get rows with maximum count

【发布时间】：2014-10-21 16:53:36

【问题描述】：

我需要执行查询以获取孩子们最喜欢的糖果，这就是我得到的：

SELECT COUNT(*) as counts,candies.name
FROM candies 
INNER JOIN kid_candy ON kid_candy.candy_id = candies.id
INNER JOIN kids ON kids.id = kid_candy.kid_id
GROUP BY candies.name

会返回：

counts  | name
--------+---------
  3     | snowbear
  3     | whiterabbit
  2     | lollipop

我只想看到

counts  | name
--------+---------
  3     | snowbear
  3     | whiterabbit

那么我的查询应该是什么？

【参考方案1】：

所以，首先让我们获取每个糖果名称的计数 (tmp_table)，然后获取所有糖果的最大计数 (max_cnt)，最后将它们放在一起并从 tmp_table 获取有计数的糖果等于max(counts) from max_cnt table...

 with tmp_table AS (
    select COUNT(*) as counts,candies.name as c_name
    from candies 
    INNER JOIN kid_candy ON kid_candy.candy_id = candies.id
    INNER JOIN kids ON kids.id = kid_candy.kid_id
    GROUP BY candies.name        
 ), 
 max_cnt AS (
    SELECT max(counts) as max_count from tmp_table
 )
 SELECT counts, c_name as candies
 FROM tmp_table 
 JOIN max_cnt on max_count = counts

【参考方案2】：

假设孩子和糖果之间存在经典的 n:m 关系，如下所述：

How to implement a many-to-many relationship in PostgreSQL?

您应该在问题中提供此类详细信息。

SELECT c.id, kc.counts, c.name
FROM  (
   SELECT candy_id AS id, count(*) AS counts
        , rank() OVER (ORDER BY count(*) DESC) AS rnk
   FROM   kid_candy
   GROUP  BY 1
   ) kc
JOIN   candies c USING (id)
WHERE  kc.rnk = 1;

这应该比使用两个 CTE 和不必要的连接的查询快得多。

要点

按candies.name 分组可能是错误的（而且成本更高）。该名称可能不是唯一的。为此使用主键列，可能是candies.id

假设参照完整性，我们根本不需要加入表 kids。

由于我们需要检查整个表，所以先聚合然后加入糖果以获得name之后更快.

您可以在聚合函数上运行窗口函数：

Get the distinct sum of a joined table column

【参考方案3】：

试试这个

SELECT * FROM
(
   SELECT COUNT(*) AS Counts,candies.name FROM candies 
     INNER JOIN kid_candy ON kid_candy.candy_id = candies.id
     INNER JOIN kids ON kids.id = kid_candy.kid_id
   GROUP BY candies.name
 ) T1 JOIN
SELECT MAX(counts) MaxCount FROM
(
   SELECT COUNT(*) AS Counts,candies.name FROM candies 
      INNER JOIN kid_candy ON kid_candy.candy_id = candies.id
      INNER JOIN kids ON kids.id = kid_candy.kid_id
   GROUP BY candies.name
) T1 ON T1.counts =T2.MaxCount

【参考方案4】：

做一个子选择

Select max(counts) 
    (select COUNT(*) as counts,candies.name from candies 
INNER JOIN kid_candy ON kid_candy.candy_id = candies.id
INNER JOIN kids ON kids.id = kid_candy.kid_id
GROUP BY candies.name)

【讨论】：

这只会得到最大计数，而不是所需的结果，但语法错误：缺少FROM 子句，缺少表别名。