MySql中GROUP_CONCAT(）的使用方法

# 根据sn、mac分组，将sn相同的数据行中的user_id、button_number分别合并成一行，并以逗号隔开
 SELECT b.sn,b.mac,b.model,GROUP_CONCAT(a.user_id) as userIds,GROUP_CONCAT(a.button_number) as buttonNumbers
                 FROM device_blood_pressure as b  LEFT JOIN device_blood_pressure_bind a ON a.mac = b.mac
                 WHERE a.user_id in
                 <foreach collection="userIds" item="item" open="(" separator="," close=")">
                #{item}
            </foreach> GROUP BY b.sn,b.mac;   

详解可看链接博客 https://baijiahao.baidu.com/s?id=1595349117525189591&wfr=spider&for=pc