无法对包含 MySql 中的聚合或子查询的表达式执行聚合函数

Posted 2023-05-09

【中文标题】无法对包含 MySql 中的聚合或子查询的表达式执行聚合函数

【英文标题】：Cannot perform an aggregate function on an expression containing an aggregate or a subquery in MySql

【发布时间】：2020-12-27 04:00:27

【问题描述】：

我想知道， 哪门课上的学生最多？

create table studies(
PNAME varchar(20),
INSTITUTE varchar(20),
COURSE varchar(20),
COURSEFEE int
);

insert into studies values
('Anand','Sabhari','PGDCA',4500),
('Altaf', 'Coit','DCA',7200),
('Juliana', 'Bdps','MCA',2200),
('Kamala', 'Pragathi','DCA',5000),
('Mary', 'Sabhari','PGDCA',4500),
('Nelson', 'Pragathi','DAP',6200),
('Patrick', 'Pragathi','DCAP',5200),
('Qadir', 'Apple','HDCA',1400),
('Ramesh', 'Sabhari','PGDCA',4500),
('Rebecca', 'Brilliant','DCAP',11000),
('Remitha', 'Bdps','DCS',6000),
('Revathi', 'Sabhari','DAP',5000),
('Vijaya', 'Bdps','DCA',4800);

查询尝试：

SELECT COURSE
FROM STUDIES
GROUP BY COURSE
HAVING COUNT(COURSE)= (SELECT MAX(COUNT(COURSE))
FROM STUDIES
GROUP BY COURSE)

【问题讨论】：

错误信息似乎不言自明

【参考方案1】：

如果你真的想让你当前的版本工作，HAVING 子句中的子查询需要更多的工作：

SELECT COURSE, COUNT(*) AS cnt
FROM STUDIES
GROUP BY COURSE
HAVING COUNT(*) = (SELECT MAX(cnt)
                   FROM (SELECT COUNT(*) AS cnt
                         FROM STUDIES
                         GROUP BY COURSE) t);

假设您希望课程数量最多，并且您不希望重复，您可以在这里使用LIMIT 查询：

SELECT COURSE, COUNT(*) AS cnt
FROM STUDIES
GROUP BY COURSE
ORDER BY COUNT(*) DESC
LIMIT 1;

或者，如果您的 mysql 版本是 8 或更高版本，您可以在此处使用 RANK 或 ROW_NUMBER：

WITH cte AS (
    SELECT COURSE, COUNT(*) AS cnt,
           RANK() OVER (ORDER BY COUNT(*) DESC) rnk
    FROM STUDIES
    GROUP BY COURSE
)

SELECT COURSE, cnt
FROM cte
WHERE rnk = 1;

【讨论】：

HAVING 子句可以简化为：HAVING COUNT(*) = (SELECT COUNT(*) FROM STUDIES GROUP BY COURSE ORDER BY COUNT(*) DESC LIMIT 1)

【参考方案2】：

你可以单独使用一个：

SELECT COURSE FROM `STUDIES` HAVING COUNT(COURSE)

如果你想要多个：

SELECT COURSE
FROM STUDIES
GROUP BY COURSE
HAVING COUNT(*) = (SELECT MAX(subject)
                  FROM (SELECT COUNT(*) AS subject
                    FROM STUDIES
                    GROUP BY COURSE) rs
                  )