Question

我有以下查询来自这篇文章count number of items in a row in mysql，它会计算一个学生在课堂上出现/缺席的次数。

    SELECT
        classlist.studentid,
        student.name,
        classStatus.name status,
        COUNT(*) presentcnt
    FROM
        classlist 
        INNER JOIN student ON classlist.studentid=student.id
        INNER JOIN classstatus ON classlist.presentid=classstatus.id
        INNER JOIN (
            SELECT
                studentid,
                max(CASE WHEN presentid=0 THEN id END)  max_0,
                max(CASE WHEN presentid=1 THEN id END)  max_1
            FROM classlist
            GROUP BY studentid
        ) s
        ON coalesce(classlist.id>least(max_0,max_1) AND classlist.id<=greatest(max_0,max_1),1) AND s.studentid=classlist.studentid
    GROUP BY classlist.studentid

这可以按预期工作，

    STUDENTID   NAME    STATUS  PRESENTCNT
    111         John    Present     1
    222         Kate    Absent      2
    333         Matt    Present     5

我想扩展查询，以便我有一个列显示学生是否参加了课程。

如果我运行独立查询，我会得到我想要的结果

    SELECT
        classlist.studentid,
        student.name,
        participatedStatus.name status,
        COUNT(*) participatedcnt
    FROM
        classlist 
        INNER JOIN student ON classlist.studentid=student.id
        INNER JOIN participatedStatus ON classlist.participatedid=participatedStatus.id
        INNER JOIN (
            SELECT
                studentid,
                max(CASE WHEN participatedid=0 THEN id END)  max_0,
                max(CASE WHEN participatedid=1 THEN id END)  max_1
            FROM classlist
            group by studentid
        ) s
        ON coalesce(classlist.id>least(max_0,max_1) 
        AND classlist.id<=greatest(max_0,max_1),1)
        AND s.studentid=classlist.studentid
    group by classlist.studentid

    STUDENTID   NAME    STATUS  PARTICIPATEDCNT
    111         John    Yes     1
    222         Kate    No      2
    333         Matt    Yes     2

但是我想将它们合并到一个查询中，所以我得到了

    STUDENTID   NAME    STATUS  PRESENTCNT  STATUS2     PARTICIPATEDCNT
    111         John    Present     1       Yes         1
    222         Kate    Absent      2       No          2
    333         Matt    Present     5       Yes         2

我对如何实现这一点感到困惑，因为我选择了数量*，我怎么能实现这个目标呢？

我正在使用的数据示例位于this fiddle及以下

    CREATE TABLE classlist
        (`id` int, `studentid` int, `subjectid` int, `presentid` int, `participatedid` int);

    CREATE TABLE student
        (`id` int, `name` varchar(4));

    CREATE TABLE subject
        (`id` int, `name` varchar(4));

    CREATE TABLE classStatus
        (`id` int, `name` varchar(8));

    CREATE TABLE participatedStatus
        (`id` int, `name` varchar(8));

    INSERT INTO classlist   (`id`, `studentid`, `subjectid`, `presentid`, `participatedid`)
    VALUES  (1, 111, 1, 1, 0),  (2, 222, 3, 0, 0),  (3, 333, 2, 1, 0),  (4, 111, 4, 0, 0),  (5, 111, 1, 1, 0),  (6, 222, 3, 0, 0),  (7, 333, 2, 1, 1),  (8, 111, 4, 0, 0),  (9, 111, 4, 0, 0),  (10, 111, 4, 0, 0), (11, 111, 1, 1, 1), (12, 333, 3, 1, 0), (13, 333, 2, 1, 1), (14, 333, 3, 1, 1);

    INSERT INTO student (`id`, `name`)
    VALUES  (111, 'John'),(222, 'Kate'),(333, 'Matt');

    INSERT INTO subject (`id`, `name`)
    VALUES  (1, 'MATH'),(2, 'ENG'),(3, 'SCI'),(4, 'GEO');

    INSERT INTO classStatus (`id`, `name`)
    VALUES  (0, 'Absent'),  (1, 'Present');

    INSERT INTO participatedStatus  (`id`, `name`)
    VALUES  (0, 'No'),(1, 'Yes');

Answer 1

SELECT
        studid,
        studname,
        status,
        presentcnt,
        status1,
        participatedcnt FROM
(SELECT
        classlist.studentid studid,
        student.name studname,
        classStatus.name status,
        COUNT(*) presentcnt
    FROM
        classlist 
        INNER JOIN student ON classlist.studentid=student.id
        INNER JOIN classstatus ON classlist.presentid=classstatus.id
        INNER JOIN (
            SELECT
                studentid,
                max(CASE WHEN presentid=0 THEN id END)  max_0,
                max(CASE WHEN presentid=1 THEN id END)  max_1
            FROM classlist
            GROUP BY studentid
        ) s
        ON coalesce(classlist.id>least(max_0,max_1) AND classlist.id<=greatest(max_0,max_1),1) AND s.studentid=classlist.studentid
    GROUP BY classlist.studentid)x
JOIN

(SELECT
        classlist.studentid,
        student.name,
        participatedStatus.name status1,
        COUNT(*) participatedcnt
    FROM
        classlist 
        INNER JOIN student ON classlist.studentid=student.id
        INNER JOIN participatedStatus ON classlist.participatedid=participatedStatus.id
        INNER JOIN (
            SELECT
                studentid,
                max(CASE WHEN participatedid=0 THEN id END)  max_0,
                max(CASE WHEN participatedid=1 THEN id END)  max_1
            FROM classlist
            group by studentid
        ) s
        ON coalesce(classlist.id>least(max_0,max_1) 
        AND classlist.id<=greatest(max_0,max_1),1)
        AND s.studentid=classlist.studentid
    group by classlist.studentid)y
ON x.studid=y.studentid

Fiddle

多个INNER JOIN子查询sql

1 个答案: