Question

我一直在研究SQLzoo问题，但在JOIN教程问题＃13中遇到最后一个问题 - 列出每个匹配的每个团队所显示的目标。

链接：http://sqlzoo.net/wiki/The_JOIN_operation

在他们提供的示例代码中，它使用了一个案例。我这样修改了它：

SELECT game.mdate, game.team1,
  CASE WHEN goal.teamid=game.team1 THEN 1 ELSE 0 END score1, game.team2,
  CASE WHEN goal.teamid=game.team2 THEN 1 ELSE 0 END score2
  FROM game, goal WHERE game.id=goal.matchid
GROUP BY game.mdate, goal.matchid, game.team1, game.team2

他们建议在score1 / score2上使用SUM函数将提供答案。我对如何在SQL中创建的这两列使用SUM函数感到困惑。

任何人都可以提供一个提示，说明如何以更好的方式编写或提及如何以更好的方式编写此SQL查询？

Answer 1

这对我有用，可以修正左连接。

SELECT mdate, team1,
SUM(CASE WHEN teamid = team1 THEN 1 ELSE 0 END) score1,
team2,
SUM(CASE WHEN teamid = team2 THEN 1 ELSE 0 END) score2
FROM game LEFT JOIN goal ON matchid = id GROUP BY mdate, matchid, team1, team2

Answer 2

嗯，你需要SUM这些列（SUM是一个聚合函数，这就是你有GROUP BY的原因。至于以更好的方式编写查询，您需要丢失旧的隐式JOIN样式并使用ANSI显式样式：

SELECT  game.mdate, 
        game.team1,
        SUM(CASE WHEN goal.teamid=game.team1 THEN 1 ELSE 0 END) score1, 
        game.team2,
        SUM(CASE WHEN goal.teamid=game.team2 THEN 1 ELSE 0 END) score2
FROM game
INNER JOIN goal 
    ON game.id=goal.matchid
GROUP BY game.mdate, goal.matchid, game.team1, game.team2

Answer 3

你的SQL非常好，你只需要一个SUM聚合函数的好例子这是我的解决方案：

 SELECT mdate,
 team1, SUM(CASE game.team1 WHEN goal.teamid THEN 1 ELSE 0 END) 'score1',
 team2, SUM(CASE game.team2 WHEN goal.teamid THEN 1 ELSE 0 END) 'score2'
 FROM game LEFT JOIN goal ON game.id=goal.matchid
 GROUP BY game.id
 ORDER BY mdate, matchid, team1, team2

注意：我的原始帖子的值不正确，需要使用LEFT JOIN代替INNER JOIN。当两个团队都有0分时，左连接会处理这种情况，内连接不会接收这些情况。

Answer 4

我认为SQLzoo问题已更新，以上答案似乎无法完全解决问题＃13。请注意，现在需要排序ORDER BY子句。

SELECT
        mdate,
        team1,
        SUM(CASE                  
            WHEN teamid=team1 THEN 1                              
            ELSE 0              
        END) as 'score1',
        team2,
        SUM(CASE                  
            WHEN teamid=team2 THEN 1                  
            ELSE 0              
        END) as 'score2'             
    FROM
        game          
    LEFT JOIN
        goal                  
            ON (
                goal.matchid = game.id                 
            )          
    GROUP BY
        mdate,
        team1,
        team2          
    ORDER BY
        mdate,
        matchid,
        team1,
        team2     ;

SQLzoo JOIN教程＃13

4 个答案: