Question

我在MySQL数据库中有两个表，课程和会话。我试图在每个月的课程中总结每个课程的参赛作品。

我可以总结每门课程的总课程数，使用此查询没问题：

   SELECT courses.CourseID, 
    SUM(IF( sessions.Duration IS NULL , 0, sessions.Duration)) AS Hrs
    FROM courses
    LEFT JOIN sessions ON courses.CourseID = sessions.CourseID
    WHERE courses.TrainerID = 113
    GROUP BY courses.CourseID

问题是并非所有课程都有会话，所以如果我试图通过在WHERE子句之后添加它来限制查询：

AND sessions.SessionDate >= '2009-06-01' AND sessions.SessionDate <= '2009-06-30'

我只参加有课程的课程，但我想要的是没有课程的课程，以显示0。

我希望这是有道理的。

有人可以帮忙吗？感谢。

Answer 1

将谓词添加到连接条件...

 SELECT courses.CourseID,     
     SUM(IF( sessions.Duration IS NULL , 0, sessions.Duration)) AS Hrs    
 FROM courses  
    LEFT JOIN sessions 
     ON courses.CourseID = sessions.CourseID
         And sessions.SessionDate 
              Between '2009-06-01' And '2009-06-30'   
 Where courses.TrainerID = 113    
 GROUP BY courses.CourseID

另外，MySql是否有Coalesce运算符？如果是这样，您可以更改为：

 SELECT courses.CourseID,     
     SUM(Coalesce(sessions.Duration, 0)) AS Hrs    
 FROM courses  
    LEFT JOIN sessions 
     ON courses.CourseID = sessions.CourseID
         And sessions.SessionDate 
              Between '2009-06-01' And '2009-06-30'   
 Where courses.TrainerID = 113    
 GROUP BY courses.CourseID

如何在第二个表可能没有相关行时限制连接

1 个答案: