Question

我有一个桌子,里面有巴士和学生签名信息:

CREATE TABLE [dbo].[BusSignupInstance](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [StudentId] [int] NOT NULL,
    [BusId] [int] NOT NULL
) ON [PRIMARY]

以下是一些抽样数据:

Id  StudentId   BusId
1   1   1
2   2   1
3   3   1
4   1   2
5   4   2
6   1   3
7   5   3

我要用公交车身份证的总数。

BusId    Num of Students
1      2 (student id 2, 3)
2      1 (student id 4)
3      2 (student id 1, 5)

结果是根据注册顺序决定的,如果一年级学生注册三次,只有最后一次是有效的。

Answer 1

SELECT  busId, COUNT(*)
FROM    (
        SELECT  busId, studentId,
                ROW_NUMBER() OVER (PARTITION BY studentId ORDER BY id DESC) rn
        FROM    busSignupInstance
        ) q
WHERE   rn = 1
GROUP BY
        busId

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