select users.user,users.name,(select count(*) from grade where users.user=grade.user) 
as total from users;

指定班级:

select users.user,users.name,(select count(*) from grade where users.user=grade.user) 
as total
from users where user like '2012%';

SELECT question.title, question.time,
(SELECT COUNT(*) FROM reply WHERE reply.q_id = question.id) AS reply_sum
FROM question

下面的问题类似