这里有一个想法：

SELECT COURSECODE, COUNT(user:_ID) FROM TABLE1, TABLE3
WHERE 
   TABLE3.user:_ID = TABLE2.course_user:_ID
AND
   TABLE2.ID_COURSE = TABLE1.ID_USER
GROUP BY COURSECODE
HAVING COUNT(TABLE2.ID_USER) < 30

请避免使用无意义或混淆的名称。如 user: _ID , ID_USER , course_user: _ID 有关此查询的更多信息，我建议您查看：
计数文档
按文档分组
有文件

试试这个：

SELECT c.COURSECODE, count(u.id) as userCount
FROM course_user cu JOIN course c on (c.id = cu.id_course) JOIN user u on (u.id = cu.id_user)
GROUP BY c.COURSECODE
HAVING userCount < 30;

看看having子句：http://www.mysqltutorial.org/mysql-having.aspx
像这样：

select Course.COURSECODE, count(course_user.ID_USER) from course
     join course_user on course_user .ID_COURSE = course._ID
     group by Course.COURSECODE
     having count(course_user.ID_USER) < 30

您可以使用：

SELECT c.COURSECODE
  FROM course_user cu 
  LEFT JOIN course c on (c.id = cu.id_course)
  LEFT JOIN user u on (u.id = cu.id_user)
 GROUP BY c.COURSECODE
 HAVING count(cu.id_user) < 30