Question

我正在开发一个玩家可以提问的系统。每个玩家被分配到不同的游戏和语言。他们也获得了奖励积分。

我需要一份报告，显示有关提出的问题数量，仍有多少问题，关闭数量以及奖金总额的数据。报告是按游戏和问题类型。

下面的查询工作正常，但现在我必须删除重复的玩家，这样每个玩家只有一个问题（他问的最后一个）。

SELECT 
q.game, 
q.language, 
q.questionType, 
COUNT(q.player) AS total, 
SUM(IF(q.status = 'Active', 1, 0)) AS active, 
SUM(IF(q.status = 'Closed', 1, 0)) AS closed, 
SUM(q.points) AS points
FROM questions AS q
LEFT JOIN players AS p ON p.id = q.asked_by 
LEFT JOIN games AS g ON g.id = p.game 
GROUP BY q.game, q.questionType

Answer 1

正如ChrisLondon指出的那样，你不需要大部分的表格。该查询的更有效版本是：

SELECT q.game, q.language, q.questionType, 
       COUNT(q.player) AS total, 
       SUM(q.status = 'Active') AS active, 
       SUM(q.status = 'Closed') AS closed, 
       SUM(q.points) AS points
FROM questions q left outer join
     (select player, max(id) as maxid
      from questions
       group by player
     ) qp
     on q.id = qp.maxid  /* don't need the player here because the ids are unique */
GROUP BY q.game, q.language, q.questionType;

许多版本的MySQL（5.6之前版本）执行优化in子查询的可怕的工作。对于处理的每一行，子查询都会重新运行（甚至可能重新编译）。将其移至from子句可以解决此问题。

这也假设短语“每个玩家只有一个问题计数（他问的最后一个）”适用于查询中的所有数量。因为它是每个玩家的最后一个，所以你可能会有一些游戏可能没有任何行，因为所有玩家都有更新的问题。如果您想要每个游戏的最后一个问题，请将子查询更改为：

SELECT q.game, q.language, q.questionType, 
       COUNT(q.player) AS total, 
       SUM(q.status = 'Active') AS active, 
       SUM(q.status = 'Closed') AS closed, 
       SUM(q.points) AS points
FROM questions q left outer join
     (select game, max(id) as maxid
      from questions
       group by game
     ) qg
     on q.id = qg.maxid  /* don't need the player here because the ids are unique */
GROUP BY q.game, q.language, q.questionType;

Answer 2

首先，你没有使用游戏桌或玩家桌上的数据，所以不需要加入它们。

SELECT 
    q.game, 
    q.language, 
    q.questionType, 
    COUNT(q.player) AS total, 
    SUM(IF(q.status = 'Active', 1, 0)) AS active, 
    SUM(IF(q.status = 'Closed', 1, 0)) AS closed, 
    SUM(q.points) AS points
FROM questions AS q
WHERE id IN (SELECT MAX(id) FROM questions GROUP BY player)
GROUP BY q.game, q.questionType

我正在做的是为每个玩家选择最后一个独特的问题，然后在其上运行查询。您的q.language中没有GROUP BY，而且它不是聚合函数。不建议你这样做。我们还建议您在群组中加入语言。

MySQL与group by不同

2 个答案: