Mysql删除重复记录，除了一个有最大时间

Question

我有一张表有多个RFID记录，每个记录都有一个名为time的列，我想要的是删除重复的RFID recods，除了一个有最长时间的记录。

表名是attendance_images（id，RFID，时间，年，月，日），我的查询如下：

DELETE t FROM attendance_images AS t LEFT JOIN (
   SELECT max( t1.time ) AS time
   FROM attendance_images AS t1
   WHERE t1.year=2016
   AND t1.month=8
   AND t1.day=4
   AND t1.time < 120000
   GROUP BY t1.RFID
) keep ON t.time = keep.time
WHERE keep.time IS NULL
AND t.year =2016
AND t.month =8
AND t.day =4
AND t.time < 120000

查询效果为（0 rows deleted. (Query took 0.0034 sec)）但表格根据条件有重复记录。

请帮助解决此问题。

Answer 1

您还需要匹配RFID：

DELETE t
    FROM attendance_images AS t LEFT JOIN
         (SELECT RFID, max( t1.time ) AS time
          FROM attendance_images AS t1
          WHERE t1.year = 2016 AND t1.month = 8 AND t1.day = 4 AND t1.time < 120000
          GROUP BY t1.RFID
         ) keep
         ON t.time = keep.time AND t.RFID = keep.RFID
WHERE keep.time IS NULL AND
      t.year = 2016 AND t.month = 8 AND t.day = 4 AND t.time < 120000;

显然，多个RFID可以拥有相同的最长时间，因此只能加入time并不起作用。

编辑：

您应该验证您确实要删除数据：

select ai.rfid, max(ai.time), min(ai.time), count(*)
from attendance_images ai
where ai.year = 2016 AND ai.month = 8 AND ai.day = 4 AND ai.time < 120000
group by ai.rfid
having min(ai.time) < max(ai.time);

我的猜测是，这将不会返回任何行，表示您没有这样的重复项。

您可以将having子句更改为having count(*) > 1，以查看是否有重复项具有相同的time值。

编辑II：

查询中值的缩进表明time存储为字符串，而不是整数。您可以尝试替换条件：

t.time < 120000

使用：

t.time + 0 < 120000