sql问题急用！求大神

[lyf_xx]

sql 语句：

select sum(select count(bu.course)*dc.period from px_course_user as bu inner join px_course as dc ON bu.course = dc.id group by bu.course) as num from dual

错误：
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'select count(bu.course)*dc.period from px_course_user as bu
inner join px_cour' at line 2

[wwwwb]

SELECT SUM(AA) FROM ( SELECT COUNT(bu.course)*dc.period AS AA FROM px_course_user AS bu INNER JOIN px_course AS dc ON bu.course = dc.id GROUP BY bu.course) A