帮我看看为什么加上desc了还是升序排序呢？

SELECT t.name as teacher_name,ifnull(count(c.name),0) as course_count
FROM teachers t
left join courses c on c.teacher_id =t.id
group by t.id
ORDER BY course_count,teacher_name desc;

每个字段后面都要指明培训方式，你这个只是最后一个字段降序，其他字段是升序

course_count 后面也加desc

每一列后面都要表明排序方式，不写，默认升序