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mysql根据多个数据库的订单表统计用户数量

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Release: 2016-06-06 09:45:48
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mysqlsql

假设db_amazon.tbl_order,db_jd.tbl_order,db_taobao.tbl_order这三张表中都存在(date, user_id)这两个字段。
同一个用户在各个表中的user_id相同
每个用户每天下单数量不限,amazon、jd、taobao也任选
amazon、jd、taobao的日下单量假设在1000W级别
求教:从执行效率的角度,如何计算出今天下过单的用户一共有多少

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