MySQL Group By If子句

您好，我继承了一个使用MySQL作为数据库的学校出勤系统，管理员每周（计划）创建出勤记录，但在这一周，记录可能会被更改或删除（实际）

我需要显示两个表之间的所有不同记录（即实际v计划）

理想情况下，我会在“Actual”表中添加一个新列并跟踪所有已删除的记录，但不允许更改模式

我创建了一个MySQL，它显示了我用来获取不同记录的模式和查询

我的问题是我不明白如何合并两行并显示 在同一行中具有当天值的记录。如果我按学生分组，则只显示第一条记录

例如，如果在“[tblPlanned]”中存在某个学生的记录，但在“tblActual”中没有，那么我需要显示该记录。我还需要显示存在于[tblActual]中但不存在于[tblPlanned]中的记录

我真正想要的是以某种方式添加一个子句（优先级），如果学生的记录在当天的任何一个表中包含一个值，则显示它，否则显示null

下面是显示数据结构和所需输出的SQL FIDLE

任何提示都会非常有用

 SELECT * FROM (
             (SELECT a.Classroom as classroom, a.Student as student, 
      MAX(case (a.DropDate) when '20160222' then a.IsAbsent else ' ' end) as 'day_1',
      MAX(case (a.DropDate) when '20160223' then a.IsAbsent else ' '  end ) as 'day_2',
      MAX(case (a.DropDate) when '20160224' then a.IsAbsent else ' '  end ) as 'day_3'
FROM Attendance a
WHERE a.DropDate IN ('20160222','20160223','20160224') AND a.classroom = '17' AND  
      NOT EXISTS( SELECT 1 
                  FROM Staging AS p 
                  WHERE p.Student = a.Student AND
                        p.IsAbsent = a.IsAbsent AND
                        p.DropDate = a.DropDate
                )
             )
           UNION 
            (SELECT t.Classroom as classroom, t.Student as student,
      MAX(case (t.DropDate) when '20160222' then t.IsAbsent else ' '  end ) as 'day_1',
      MAX(case (t.DropDate) when '20160223' then t.IsAbsent else ' '  end ) as 'day_2',
      MAX(case (t.DropDate) when '20160224' then t.IsAbsent else ' '  end ) as 'day_3'
 FROM Staging t
 WHERE t.DropDate IN ('20160222','20160223','20160224') AND t.classroom = '17'AND  
       NOT EXISTS( SELECT 1
                   FROM Attendance AS u
                   WHERE u.Student = t.Student AND 
                         u.IsAbsent = t.IsAbsent AND 
                         u.DropDate = t.DropDate
                 )
            )
    ) tbl  ORDER BY  classroom, student

---

你在找这样的东西吗？

LEFT JOIN

或

RIGHT JOIN

子查询取决于您希望哪个子查询具有优先级，然后使用

IFNULL

函数

SELECT tbl2.classroom, tbl2.student, IFNULL(tbl2.day_1, tbl1.day_1) day_1
  ,IFNULL(tbl2.day_2, tbl1.day_2) day_2, IFNULL(tbl2.day_3, tbl1.day_3) day_3 
FROM 
((SELECT
  a.Classroom as classroom,
  a.Student as student,
  MAX( case (a.DropDate)
       when '20160222'
       then a.IsAbsent
       else ' '  end ) as 'day_1',
  MAX( case (a.DropDate)
       when '20160223'
       then a.IsAbsent
       else ' '  end ) as 'day_2',
  MAX( case (a.DropDate)
       when '20160224'
       then a.IsAbsent
       else ' '  end ) as 'day_3'
  FROM Attendance a
  WHERE a.DropDate IN ('20160222','20160223','20160224')
  AND a.classroom = '17'
  AND  NOT EXISTS 
      ( SELECT 1
        FROM Staging AS p
        WHERE p.Student = a.Student
          AND p.IsAbsent = a.IsAbsent
          AND p.DropDate = a.DropDate
      )
) as tbl1
RIGHT JOIN
(SELECT
  t.Classroom as classroom,
  t.Student as student,
  MAX( case (t.DropDate)
       when '20160222'
       then t.IsAbsent
       else ' '  end ) as 'day_1',
  MAX( case (t.DropDate)
       when '20160223'
       then t.IsAbsent
       else ' '  end ) as 'day_2',
  MAX( case (t.DropDate)
       when '20160224'
       then t.IsAbsent
       else ' '  end ) as 'day_3'
FROM Staging t
WHERE t.DropDate IN ('20160222','20160223','20160224')
  AND t.classroom = '17'
  AND  NOT EXISTS 
      ( SELECT 1
        FROM Attendance AS u
        WHERE u.Student = t.Student
          AND u.IsAbsent = t.IsAbsent
          AND u.DropDate = t.DropDate
      )
)tbl2 ON tbl1.classroom = tbl2.classroom and tbl1.student = tbl2.student) 


ORDER BY  classroom, student

---

您能在实际问题中向我们展示您的原始表格吗？谢谢@TimBiegeleisen我的模式在这里可用。您能更好地描述您正在寻找的列和条件的确切输出吗？谢谢，这太完美了！！有没有办法确定它来自哪个表。例如，我可以在day_1或day_2列中显示它是tbl1还是tbl2吗？再次感谢您可以将其更改为

完全联接

并选择所有字段：

选择…，tbl1.day_1，tbl2.day_1，tbl1.day_2，tbl2.day_2，…

，然后您就知道哪个表是空的，哪个表不是空的。