如何计算 mysql 子查询中的行数

Question

I have two tables course and registered_course

course table

crid	crname	crlevel
1	math	senior
2	english	senior
3	physics	senior

Registered course table

id	crid	student_id
1	2	25
2	2	26
3	3	23
4	3	24
5	3	27

so i want to achieved this result the first table join with second table and a count of students that registered a subject just like below thanks

crname	crlevel	number_of_student
math	senior	2
physic	senior	3

Answer 1

SELECT table1.crname, table1.crlevel, count(table2.studentid)
FROM table1 
INNER JOIN table2 ON table1.crid = table2.crid
GROUP BY table1.crname, table1.crlevel