Question

我在下面写了这个MySQL查询。

SELECT doors.mac_addr,
       readers.reader_name,
       reader_records.value,
       reader_records.time_change
  FROM (building.readers readers
        INNER JOIN building.doors doors
           ON (readers.gateway_id = doors.id))
       INNER JOIN building.reader_records reader_records
          ON (reader_records.reader_id = readers.id)
 WHERE (doors.mac_addr = 'B99A88')
ORDER BY reader_records.time_change DESC

它产生以下结果;

(mac_addr, reader_name, value, time_change) VALUES
('B99A88', 'name_8', 1, '7/7/2016 7:21:48 PM'), 
('B99A88', 'own__detect_1', 1, '6/21/2016 1:30:00 PM'), 
('B99A88', 'own__temperature_1', 37.4, '5/4/2016 6:23:03 PM'), 
('B99A88', 'own__temperature_1', 29.4, '5/4/2016 6:19:33 PM'), 
('B99A88', 'own__temperature_1', 28.4, '5/4/2016 6:17:32 PM'), 
('B99A88', 'own__temperature_1', 27.4, '5/4/2016 6:04:08 PM'), 
('B99A88', 'own__temperature_1', 21.4, '5/4/2016 3:11:42 PM'), 
('B99A88', 'own__detect_1', 0, '4/20/2016 3:22:23 PM'), 
('B99A88', 'own__detect_1', 1, '4/15/2016 5:39:52 PM'), 
('B99A88', 'own__detect_1', 0, '4/15/2016 5:39:46 PM'), 
('B99A88', 'own__detect_1', 1, '4/11/2016 5:34:00 PM'), 
('B99A88', 'own__detect_1', 1, '4/11/2016 5:33:00 PM'), 
('B99A88', 'own__detect_1', 0, '4/11/2016 5:33:00 PM'), 
('B99A88', 'own__temperature_1', 28.4, '4/10/2016 9:20:20 PM'), 
('B99A88', 'own__temperature_1', 32.5, '4/10/2016 9:00:00 PM'), 
('B99A88', 'own__temperature_1', 34.2, '4/10/2016 11:29:00 AM')

但是，它并不是我想要的，因为它检索每个reader_name的所有记录。我想要的只是检索每个reader_name的最后一条记录。我想要的所需查询应生成此输出;

(mac_addr, reader_name, value, time_change) 
('B99A88', 'name_8', 1, '7/7/2016 7:21:48 PM'), 
('B99A88', 'own__detect_1', 1, '6/21/2016 1:30:00 PM'), 
('B99A88', 'own__temperature_1', 37.4, '5/4/2016 6:23:03 PM'),

如何修改我的查询以获得所需的结果？

编辑：如果需要每个实体的最后X条记录怎么办？说，最后2条记录。如果需要最后2条记录，则所需的结果如下所示;

(mac_addr, reader_name, value, time_change) VALUES
('B99A88', 'name_8', 1, '7/7/2016 7:21:48 PM'), 
('B99A88', 'own__detect_1', 1, '6/21/2016 1:30:00 PM'), 
('B99A88', 'own__detect_1', 0, '4/20/2016 3:22:23 PM'), 
('B99A88', 'own__temperature_1', 37.4, '5/4/2016 6:23:03 PM'), 
('B99A88', 'own__temperature_1', 29.4, '5/4/2016 6:19:33 PM')

Answer 1

试试这个：

SELECT t1.*,
       IF(@rn = reader_name, @rowno := @rowno + 1, @rowno := 1) AS rowno,
       @rn := reader_name
FROM (
    SELECT doors.mac_addr,
           readers.reader_name,
           reader_records.value,
           reader_records.time_change
      FROM (building.readers readers
            INNER JOIN building.doors doors
               ON (readers.gateway_id = doors.id))
           INNER JOIN building.reader_records reader_records
              ON (reader_records.reader_id = readers.id)
     WHERE (doors.mac_addr = 'B99A88')
    ORDER BY readers.reader_name, reader_records.time_change DESC
) t1
CROSS JOIN (SELECT @rn := null, @rowno := 0) t2
HAVING rowno = 1
-- HAVING rowno <= 2

<强> 编辑：

SELECT mac_addr, reader_name, value, time_change
FROM (
    SELECT t1.*,
           IF(@rn = reader_name, @rowno := @rowno + 1, @rowno := 1) AS rowno,
           @rn := reader_name
    FROM (
        SELECT doors.mac_addr,
               readers.reader_name,
               reader_records.value,
               reader_records.time_change
          FROM (building.readers readers
                INNER JOIN building.doors doors
                   ON (readers.gateway_id = doors.id))
               INNER JOIN building.reader_records reader_records
                  ON (reader_records.reader_id = readers.id)
         WHERE (doors.mac_addr = 'B99A88')
        ORDER BY readers.reader_name, reader_records.time_change DESC
    ) t1
    CROSS JOIN (SELECT @rn := null, @rowno := 0) t2
) t
WHERE rowno <= 2

Answer 2

如果每组的最新记录足够，可以使用简单的解决方案：

SELECT d.mac_addr,
       r.reader_name,
       SUBSTRING_INDEX(GROUP_CONCAT(rr.value ORDER BY rr.time_change DESC), ',', 1) AS value
       SUBSTRING_INDEX(GROUP_CONCAT(rr.time_change ORDER BY rr.time_change DESC), ',', 1) AS time_change
  FROM (building.readers r
        INNER JOIN building.doors d
                ON (r.gateway_id = d.id))
 INNER JOIN building.reader_records rr
         ON (rr.reader_id = r.id)
 WHERE (d.mac_addr = 'B99A88')
 GROUP BY d.mac_addr, r.reader_name
 ORDER BY d.mac_addr, r.reader_name;

通过一些调整，它也可以实现N个最新行，尽管解决方案将远非不错。请参阅this blog post。

Answer 3

STOP PRESS ：MySQL不支持窗口函数！抱歉，这对你不起作用。

尝试使用像ROW_NUMBER这样的窗口函数：
SELECT mac_addr, reader_name, VALUE, time_change FROM (SELECT doors.mac_addr, readers.reader_name, reader_records.VALUE, reader_records.time_change, ROW_NUMBER() OVER ( PARTITION BY doors.mac_addr, readers.reader_name ORDER BY reader_records.time_change DESC ) rowno FROM (building.readers readers INNER JOIN building.doors doors ON (readers.gateway_id = doors.ID)) INNER JOIN building.reader_records reader_records ON (reader_records.reader_id = readers.ID) WHERE (doors.mac_addr = 'B99A88')) WHERE rowno = 1 ORDER BY reader_records.time_change DESC
窗口函数是ANSI标准的一部分，因此它们在数据库中的工作方式相同，我的例子来自Oracle。一旦掌握了基础知识，就会有其他有用的窗口函数，例如LEAD和LAG，您可以使用这些函数来判断门是否打开了多长时间。

Answer 4

如果您使用SQL USE TOP 1获取TOP 1行

SELECT TOP 1 doors.mac_addr,
           readers.reader_name,
           reader_records.value,
           reader_records.time_change
      FROM (building.readers readers
            INNER JOIN building.doors doors
               ON (readers.gateway_id = doors.id))
           INNER JOIN building.reader_records reader_records
              ON (reader_records.reader_id = readers.id)
     WHERE (doors.mac_addr = 'B99A88')
    ORDER BY reader_records.time_change DESC

在Mysql中使用LIMIT

SELECT doors.mac_addr,
           readers.reader_name,
           reader_records.value,
           reader_records.time_change
      FROM (building.readers readers
            INNER JOIN building.doors doors
               ON (readers.gateway_id = doors.id))
           INNER JOIN building.reader_records reader_records
              ON (reader_records.reader_id = readers.id)
     WHERE (doors.mac_addr = 'B99A88')
    ORDER BY reader_records.time_change DESC LIMIT 1

Answer 5

一种解决方案是使用子查询返回每个record_id的最大（最新）time_change：

select id, max(time_change) as latest_time_change
from reader_records
group by id

然后，您可以在原始查询中插入上一个查询：

select
  doors.mac_addr,
  readers.reader_name,
  reader_records.value,
  reader_records.time_change
from
  building.readers readers INNER JOIN building.doors doors
  ON readers.gateway_id = doors.id
  INNER JOIN (
    select id, max(time_change) as latest_time_change
    from reader_records
    group by id
  ) mr
  ON reader_records.reader_id=mr.reader_id
  INNER JOIN building.reader_records reader_records
  ON mr.reader_id = readers.id AND mr.latest_time_change=readers.time_change
 where
   doors.mac_addr = 'B99A88'
 order by
  reader_records.time_change DESC

仅检索每个实体的最后一条记录而不是所有记录

5 个答案: