with a as(
    select 
        ct.course_id,
        ct.course_name,
        round(sum(bt.if_sign)*100/sum(bt.if_vw),2) as sign_rate
    from 
        behavior_tb as bt
        left join course_tb as ct on bt.course_id=ct.course_id
    group by 
        ct.course_id,
        ct.course_name
)
select * from a