按学生数分组的日期和的MySQL连接即使行连接结果为0，也要返回值

如何解决按学生数分组的日期和的MySQL连接即使行连接结果为0，也要返回值

如何使没有结果（匹配项）的学生在此查询中也显示0 TotalHours。

添加了我在dbfiddle及以下版本中所需要的真实示例：

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我想在查询中显示记录，以便它显示ID 2593，它没有任何出勤记录，因此我希望它在totalhours列中返回0。

select
   students.StudentNumber,IFNULL(round(sum(TimestampDiff(minute,Time_Start,Time_End)) / 60,2),0) as TotalHours 
from
   students 
   left join
      attendance 
      on students.StudentNumber = attendance.StudentNumber 
where
   time_start between '2019-01-01' and '2020-09-12' 
Group by
   students.StudentNumber

https://www.db-fiddle.com/f/iZDMnePyCvzoW82eScXoRF/0

解决方法

您可以将time where子句放在子选择项中以便出勤，然后获得结果。

您应仅将SELECT * FROM attendance WHERE time_start BETWEEN '2019-01-01' AND '2020-09-12'替换为实际需要的列

---

查询＃1

SELECT 
    students.StudentNumber,IFNULL(ROUND(SUM(TIMESTAMPDIFF(MINUTE,Time_Start,Time_End)) / 60,2),0) AS TotalHours
FROM
    students
        LEFT JOIN
    attendance ON students.StudentNumber = attendance.StudentNumber
WHERE
    time_start BETWEEN '2019-01-01' AND '2020-09-12' or time_start IS NULL
GROUP BY students.StudentNumber;

| StudentNumber | TotalHours |
| ------------- | ---------- |
| 2591          | 7.48       |
| 2592          | 6.50       |
| 2593          | 0.00       |

---

查询＃2

SELECT 
    s.StudentNumber,0) AS TotalHours
    
FROM
    students s
        LEFT JOIN
    ( SELECT * FROM attendance WHERE
    time_start BETWEEN '2019-01-01' AND '2020-09-12') a ON s.StudentNumber = a.StudentNumber
GROUP BY s.StudentNumber;

| StudentNumber | TotalHours |
| ------------- | ---------- |
| 2591          | 7.48       |
| 2592          | 6.50       |
| 2593          | 0.00       |

---

View on DB Fiddle