mysql计算订单时间、订单数量

mysql 怎么统计七天内的订单，和每日的订单

select count(distinct user_id)

from

(select user_id from db_amazon.tbl_order where `date` = ...

union all

select user_id from db_jd.tbl_order where `date` = ...

union all

select user_id from db_taobao.tbl_order where `date` = ...

)

登陆后获取userid，然后指定订单表user=刚才的

select * from order a,user b where a.id=b.id and b.userid='XXX'

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