联合查询 UNION ALL

多个表 联合查询 UNION ALL汇总，（各表的字段数量、类型必须相同）

查询总数时，一定要在最后写个“别名”，否则不正确（我自己暂时这么叫，因为我也不知道原因）

select count(*) as total from (
SELECT order_id FROM `member_orders`
UNION ALL
SELECT id FROM `orders`
UNION ALL
SELECT id FROM `phone_orders`
) c

SELECT order_id,receiver_name,receiver_address FROM `member_orders` where order_id=10
UNION ALL
SELECT id,username,acceptadd FROM `orders` where id=10
UNION ALL
SELECT id,username,address FROM `phone_orders` where id=10