选择最大（计数）MySQL

Question

I am new to MySQL, and I have a task to do right now where I have three tables:

1. students(id,name)
2. courses(id,name)
3. grades(id, student_id (FK), course_id(FK), grade)

I am supposed to

fetch the name of the course that is the most registered by students and if there is any conflict or ties with other course, retrieve the course after sorting ascendingly.

I tried several queries, but they are not 'efficient enough'

SELECT course.name FROM (
SELECT CI ,MAX(Total) FROM 
(
    SELECT course_id as CI,COUNT(*) AS Total 
    FROM grades
    GROUP BY course_id ASC
) AS Results

) AS x

INNER JOIN courses ON x.CI = courses.id

And

SELECT courses.name FROM (
SELECT course_id, COUNT(*) AS how_many
    FROM grades
    GROUP BY course_id ASC
    HAVING how_many = (
        SELECT COUNT(*) AS how_many
            FROM grades
            GROUP BY course_id
            ORDER BY how_many DESC
            LIMIT 1
    )
    LIMIT 1
) AS X
JOIN courses ON X.course_id=courses.id

Is there any more efficient query?

Answer 1

Both your query attempts look logically incorrect to me. You should be joining courses to grades to obtain the number of students enrolled in each course. Regarding efficiency, the RANK analytic function is one of the most performant options, assuming you are running MySQL 8+:

WITH cte AS (
    SELECT c.id, c.name, RANK() OVER (ORDER BY COUNT(*) DESC, c.name) rnk
    FROM courses c
    INNER JOIN grades g ON g.course_id = c.id
    GROUP BY c.id, c.name
)

SELECT id, name
FROM cte
WHERE rnk = 1;

On earlier versions of MySQL, we can use a LIMIT query:

SELECT c.id, c.name
FROM courses c
INNER JOIN grades g ON g.course_id = c.id
GROUP BY c.id, c.name
ORDER BY COUNT(*) DESC, c.name
LIMIT 1;

Answer 2

You can use the ORDER BY clause with the LIMIT clause to get what you need, without aggregating twice:

WITH enrollments AS (
    SELECT course_id, COUNT(DISTINCT student_id) AS num_enrollments
    FROM grades
    GROUP BY course_id
)
SELECT * 
FROM       enrollments e
INNER JOIN courses c
        ON e.course_id = c.id
ORDER BY e.num_enrollments DESC, c.name ASC
LIMIT 1

The subquery will get you the enrollments by aggregating on the students, then it is joined with the courses to use the course name.

Data is then ordered by:

• number of enrollments descendent
• course name ascendent

and only the first row is considered.