MYSQL中选择多个值的MAX函数

Question

我正在使用 MYSQL 来获取助教拥有的最大学生人数。 通过这个查询，我得到了所有的助教和每个助教的学生人数

SELECT TA.Name as Name, COUNT(Lecture.Student) AS studentcount 
FROM TA 
JOIN Lecture 
WHERE TA.TA_PUID = Lecture.TA 
GROUP BY TA.Name
ORDER BY studentcount DESC;

我的输出是这个

David    2 
Justin   2
Matt     2
Jennifer 1
Hannah   1
Timothy  1
Bob      1

我只想获得具有最大学生价值的助教。 我怎样才能做到这一点？ 在这种情况下，我希望我的输出是

David  2 
Justin 2
Matt   2

Answer 1

您可以使用rank() ：

SELECT t.*
FROM (SELECT TA.Name as Name, COUNT(*) AS studentcount,
             RANK() OVER (ORDER BY COUNT(*) DESC) as seqnum
      FROM TA JOIN
           Lecture l
           ON TA.TA_PUID = l.TA
      GROUP BY TA.Name
    ) t
WHERE seqnum = 1;

请注意， JOIN后面应该跟一个ON子句而不是WHERE子句。

Answer 2

你没有提到 MySQL 的版本。 在 MySQL 8.x 中，您可以使用 CTE（公用表表达式）来预先计算值。 然后，过滤很容易。

例如：

with
a as (
  SELECT TA.Name as Name, COUNT(Lecture.Student) AS studentcount 
  FROM TA 
  JOIN Lecture 
  WHERE TA.TA_PUID = Lecture.TA 
  GROUP BY TA.Name
)
select * from a where studentcount = (select max(studentcount) from a)

Answer 3

在 MySQL 8 之前，您必须执行以下操作：

SELECT TA.Name as Name, COUNT(Lecture.Student) AS studentcount 
FROM TA INNER JOIN Lecture ON TA.TA_PUID = Lecture.TA 
GROUP BY TA.Name
HAVING studentCount = (
   SELECT COUNT(Student) AS studentcount 
   FROM Lecture
   GROUP BY TA
   ORDER BY studentcount DESC
   LIMIT 1
);

如果您使用的是 MySQL 8+，那么窗口函数和公用表表达式绝对是首选。