mysql - Sql 查询分组和限制

Question

我有以下 sql 查询，它根据 topicName 列进行分组(它还进行一些除法运算)。 我想为每个分组主题获取 2 行，而不是全部。

    SELECT wwt.topicName, t.topic_cnt as sumOfWordsInTopic,
               wwt.word, wwt.wordCount,
               (wwt.wordCount / t.topic_cnt) AS wordProbability
         FROM weightallofwordsintopic  as wwt JOIN
             (SELECT  topicName, sum(wordCount) AS topic_cnt
              FROM weightallofwordsintopic 
              GROUP BY topicName
            ) t 
         ON wwt.topicName = t.topicName

主题表中的所有单词的权重如下；

topicName | word | wordCount
---
topic0  | word1     | 10  
topic0  | word2     | 20  
topic0  | word3     | 30  
topic0  | word4     | 40  
topic0  | word5     | 50  
topic0  | word6     | 60 

topic1  | word7     | 10  
topic1  | word8     | 20  
topic1  | word9     | 30  
topic1  | word10    | 40  
topic1  | word11    | 50  
topic1  | word12    | 60 

topic2  | word13    | 10  
topic2  | word14    | 20  
topic2  | word15    | 30  
topic2  | word16    | 40  
topic2  | word17    | 50 
topic2  | word18    | 60 

我希望输出为(根据其权重排序，但在这里我只是放置一个示例(上面的选择查询返回一些不同的列)) 我想根据每个分组 topicName 在列中的权重将上述查询限制为 2 行。

topicName | word | wordCount

topic0  | 1     | 60  
topic0  | 1     | 50  

topic1  | 1     | 60 
topic1  | 1     | 50  

topic2  | 1     | 60 
topic2  | 2     | 50  

Answer 1

在 MySQL 中，最简单的方法可能是使用变量:

SELECT t.*
FROM (SELECT wwt.topicName, t.topic_cnt as sumOfWordsInTopic, wwt.word, wwt.wordCount,
              (wwt.wordCount / t.topic_cnt) AS wordProbability,
              (@rn := if(@t = wwt.topicName, @rn + 1,
                         if(@t := wwt.topicName, 1, 1)
                        )
              ) as rn
      FROM weightallofwordsintopic  as wwt JOIN
           (SELECT topicName, sum(wordCount) AS topic_cnt
            FROM weightallofwordsintopic 
            GROUP BY topicName
           ) t 
           ON wwt.topicName = t.topicName CROSS JOIN
           (SELECT @t := '', @rn := 0) params
      ORDER BY wwt.topicName, wwt.wordCount DESC
     ) t
WHERE rn <= 2;