[英]Group by in DB2
我有一个查询要为 DB2 中的用户获取最近的未来医生预约。
SELECT user_id,
appointment_id,
date
FROM (SELECT user_id,
appointment_id,
date
FROM doctors_appointment
WHERE date > current_timestamp
AND user_id IN (23, 24, 25)
ORDER BY date ASC ) as tmp_appointment
GROUP BY user_id
但是在 group by 语句中包含约会 ID 和日期会引发错误,这意味着它要求在 group by 中包含所有 select 字段。
如果我按结果将所有字段包含在组中,将是这样的:
user_id appointment_id date
-------- ---------------- -------
23 450 2011-07-20
23 451 2011-07-26
25 452 2011-07-18
但我需要如下结果:
user_id appointment_id date
-------- -------------- -------
23 450 2011-07-20
25 452 2011-07-18
我认为这可以解决问题:
SELECT D.User_ID, D.Appointment_ID, D.Date
FROM Doctors_Appointment AS D
JOIN (SELECT User_ID, MIN(Date) AS Apt_Date
FROM Doctors_Appointment
WHERE Date > current_timestamp
AND User_ID IN (23, 24, 25)
GROUP BY User_ID) AS T
ON T.User_ID = D.User_ID AND T.Apt_Date = D.Date;
如果给定用户在同一天有两个约会,它将为给定用户生成两行。
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