[英]MySQL - SELECT MAX(), and get corresponding fields
任务是:获取每个员工的 ID 和他工作的最后一个部门的 ID 的列表。 这变得越来越复杂,因为一个人可以同时在不同的部门工作,所以我们需要让他的最后一个部门拥有最大的比率。
桌子:
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 值表示他的合同是今天生效的。
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 选择带有相应字段的 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,结果中有所有相应的行:
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 MAX(rate) 的对应行? 我认为 HAVING 可能会有所帮助,但我仍然不知道那里必须有什么。
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。
你可以试试下面的查询:
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)
我认为这个查询对你有用。
SELECT ID_employee, ID_department, end_date, MAX(rate)
FROM test_max
GROUP BY ID_employee
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