MySQL - SELECT MAX(), 并获取相应字段

Question

任务是:获取每个员工的ID和他上一个工作部门的ID的列表。它变得越来越复杂，因为一个人可以同时在不同的部门工作，所以我们需要得到他最后一个拥有最大比率的部门。

表格:

 ID_employee| ID_department | end_date  |   rate
 1            22              2016-01-01    1
 2            25              NULL          0.3
 2            27              NULL          1
 3            22              2013-12-12    0.5
 3            22              2014-05-05    0.5

end_date是员工工作的最后一天，NULL值表示他的契约(Contract)在今天生效。

结果必须如下所示:

ID_employee | ID_department | end_date  |   rate
1             22              2016-01-01    1
2             27              NULL          1
3             22              2014-05-05    0.5

我发现了如何使用 join 来选择具有相应字段的 max():

SELECT table.id_employee, id_department
FROM table
JOIN (  SELECT id_employee,
    IF (MAX( end_date IS NULL ) = 1 , "0000-00-00",  MAX( end_date )) as max_end_date
    FROM table GROUP BY id_employee) maxs ON maxs.id_employee = table.id_employee
WHERE maxs.max_end_date = IFNULL(table.end_date, "0000-00-00")
GROUP BY table.id_employee

但是，结果中有所有对应的行:

ID_employee | ID_department | end_date  |   rate
1             22              2016-01-01    1
2             25              NULL          0.3
2             27              NULL          1
3             22              2014-05-05    0.5

问题是，如何不仅获得与 MAX(end_date) 对应的行，还获得与 MAX(rate) 对应的行？我认为 HAVING 可能会有所帮助，但我仍然不知道那里到底有什么。

也许还有其他方法可以更好地解决问题，因为这个查询大约需要 16 秒，而表有大约 30 000 行。

Answer 1

你能试试下面的查询吗:

SELECT  T1.ID_employee,  
        T1.ID_department, 
        CASE WHEN maxs.max_end_date = "0000-00-00" THEN NULL ELSE maxs.max_end_date END AS end_date, 
        T1.rate
FROM TestTable T1
JOIN (  SELECT  id_employee,
                MAX(ID_department) AS ID_department,
                IF (MAX( end_date IS NULL ) = 1, "0000-00-00",  MAX( end_date )) AS max_end_date
        FROM TestTable 
        GROUP BY id_employee ) maxs ON maxs.id_employee = T1.id_employee AND maxs.ID_department = T1.ID_department
WHERE maxs.max_end_date = IFNULL(T1.end_date, "0000-00-00")
GROUP BY T1.id_employee

请找到 Live Demo

更新:

根据评论，以下查询有助于实现结果:

SET @CurrentDate := CURDATE();

SELECT T2.ID_employee, 
       T2.ID_department, 
       CASE WHEN MR.Max_end_date = @CurrentDate THEN NULL ELSE T2.end_date END AS end_date, 
       MR.MaxRate AS rate
FROM TestTable T2 
JOIN (
    SELECT T1.ID_employee, MAX(T1.rate) AS MaxRate, MD.Max_end_date
    FROM TestTable T1
    JOIN (
        SELECT  ID_employee,
                MAX(CASE WHEN end_date IS NULL THEN @CurrentDate ELSE end_date END) AS Max_end_date
        FROM TestTable 
        GROUP BY ID_employee
        ) MD ON MD.ID_employee = T1.ID_employee
    WHERE MD.Max_end_date = IFNULL(T1.end_date, @CurrentDate)
    GROUP BY T1.ID_employee
) MR ON MR.ID_employee = T2.ID_employee AND MR.MaxRate = T2.rate 
WHERE MR.Max_end_date = IFNULL(T2.end_date, @CurrentDate)

Working Demo