Question

链接为http://sqlzoo.net/wiki/The_JOIN_operation。问题13。现在我可以列出所有匹配项，只要代码中至少有一个目标：

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 JOIN goal ON (id = matchid)
    GROUP BY id
    ORDER BY mdate, matchid, team1, team2

然而，有些比赛的得分为0：0。我的代码无法显示这些游戏，也无法找到其他可用的解决方案。真的希望有人可以帮我解决这个问题。

Answer 1

托盘LEFT JOIN代替JOIN。因为即使没有目标，你也需要展示所有游戏。

Answer 2

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 (id = matchid)
    GROUP BY mdate,team1,team2
    ORDER BY mdate, matchid, team1, team2

你想在SELECT中没有聚合函数的GROUP BY列，并使用LEFT JOIN。

Answer 3

SELECT G.mDate
      ,G.Team1
      ,ISNULL( (SELECT COUNT(*) FROM goal WHERE teamid = G.Team1), 0) AS Team1_Goals 
      ,G.Team2
      ,ISNULL( (SELECT COUNT(*) FROM goal WHERE teamid = G.Team2), 0) AS Team2_Goals      
FROM Game G 
WHERE EXISTS (SELECT 1
              FROM goal 
              WHERE matchid = G.matchid)

在SQLZoo中加入SQL＃13的解决方案

3 个答案: