Question

这是我当前的查询：

SELECT Takes_T.sec_id AS 'Section ID',
Takes_T.semester AS 'Semester',
Takes_T.year AS 'Year',
COUNT(Takes_T.takes_id) AS 'Enrollment'


From Takes_T

Group By Takes_T.year, Takes_T.semester, Takes_T.sec_id
ORDER BY Takes_T.year, Takes_T.semester DESC, Takes_T.sec_id,
Takes_T.course_id

;

这产生以下结果：
（我为糟糕的格式道歉。我无法弄清楚如何在表中得到这个）

Section ID | Semester | Year | Enrollment
     1     | Spring   | 2014 | 12922
     2     | Spring   | 2014 |  1434
     3     | Spring   | 2014 |   322
     1     | Fall     | 2014 | 12590
     2     | Fall     | 2014 |  2732

我需要做的是显示每个部分的MAX注册（部分是部分ID，学期和年份的组合）。所以我需要的是：

Section ID  Semester   Year    Enrollment
    1       Spring     2014      12922
    1        Fall      2014      12590

如何修改查询以显示每个部分的MAX？

Answer 1

我不知道您正在使用哪种RDBMS，但以下内容应与MS SQL Server一起使用：

SELECT
    sec_id AS 'Section ID',
    semester AS 'Semester',
    year AS 'Year',
    COUNT(takes_id) AS 'Enrollment'
FROM Takes_T t
WHERE sec_id = (
    SELECT TOP 1 sec_id
    FROM Takes_T
    WHERE year = t.year
    AND semester = t.semester
    GROUP BY year, semester
    ORDER BY count(*) DESC
)
GROUP BY year, semester, sec_id
ORDER BY year, semester DESC, sec_id

编辑：这对MySQL来说是一样的：

SELECT
    sec_id AS 'Section ID',
    semester AS 'Semester',
    year AS 'Year',
    COUNT(takes_id) AS 'Enrollment'
FROM Takes_T t
WHERE sec_id = (
    SELECT sec_id
    FROM Takes_T
    WHERE year = t.year
    AND semester = t.semester
    GROUP BY year, semester
    ORDER BY count(*) DESC
    LIMIT 1
)
GROUP BY year, semester, sec_id
ORDER BY year, semester DESC, sec_id

Answer 2

您可以执行嵌套选择：

SELECT max(Enrollment), SectionId, Semester, Year 
FROM
(SELECT 
   Takes_T.sec_id AS 'SectionID',
   Takes_T.semester AS 'Semester',
   Takes_T.year AS 'Year',
   COUNT(Takes_T.takes_id) AS 'Enrollment'
FROM Takes_T
GROUP BY Takes_T.year, Takes_T.semester, Takes_T.sec_id) a
GROUP BY SectionId, Semester, Year

Answer 3

我明白了。这是我使用的代码：

SELECT Semester,
Year,
MIN(Enrollment) AS 'Min Enrollment',
MAX(Enrollment) AS 'Max Enrollment' 

FROM(
SELECT Takes_T.sec_id AS 'Section_ID',
Takes_T.semester AS 'Semester',
Takes_T.year AS 'Year',
COUNT(Takes_T.takes_id) AS 'Enrollment'


From Takes_T

Group By Takes_T.year, Takes_T.semester, Takes_T.sec_id) MaxE
GROUP BY Year, Semester
ORDER BY Semester DESC
;

Answer 4

如果可以使用临时表，可以试试这个：

CREATE TEMPORARY TABLE IF NOT EXISTS enrollment_count AS (
    SELECT
        sec_id,
        semester,
        year,
        COUNT(takes_id) AS 'enrollment'
    FROM Takes_T
    GROUP BY year, semester, sec_id
);
SELECT
    sec_id,
    semester,
    year,
    enrollment
FROM enrollment_count ec
WHERE enrollment = (
    SELECT MAX(enrollment)
    FROM enrollment_count sub
    WHERE sub.semester = ec.semester
    AND sub.year = ec.year
)

根据以前的COUNT（）显示多个组的最大值

4 个答案: