说起mysql数据库，我们可以很熟练的使用left join

经常说的小表驱动大表，以左边的表的数据为基准

今天要说是的leftjoin如何只取得右表中的一条数据

有人说我可以用php来实现

但是有点麻烦，看看单纯用sql语句能不能实现

select o.order_sn,from_unixtime(o.add_time) from user u left join order o on u.username=o.username where u.username='13812345678';

以上为查询所有的订单

SELECT
o.order_sn,from_unixtime(o.add_time)
FROM
    user as u
LEFT JOIN (
    SELECT order_sn,username,add_time
    FROM
        order
    WHERE
        1 and username in('13812345678','13112345678')
    ORDER BY
        add_time ASC
) as o ON o.username = u.username
WHERE
    1 and u.username in('13812345678','13112345678')
GROUP BY
    u.username

以上sql为查询时间最早的数据

SELECT 
 a.username,from_unixtime(a.create_time),b.order_sn,from_unixtime(b.add_time)
FROM
  user a 
  LEFT JOIN 
    (SELECT *FROM  order WHERE id IN   (SELECT max(id)   FROM  order  where username in('13812345678','13112345678')  GROUP BY username ) ) b ON b.`username` = a.`username` where b.username in('13812345678','13112345678')

以上sql为查询时间最新数据

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