mysql左链接 left join

//主表关联从表，主表的数据全部显示，从表要看条件满足才显示

//例如：主表sys_user,从表分别是edu_user,edu_user_coupon

select e.*,sum(c.number)as number from sys_user s

left join edu_user e on e.USER_ID=s.edu_user_id

left join edu_user_coupon c on c.user_id=s.id

where s.id='30836'