Question

您好我有以下查询显示以下结果：

SELECT 
  meter.meter_serial,
  her060.node_address,
  her060.report_time,
  her060.node_reading 
FROM
  meter,
  her060 
WHERE meter.node_address = her060.node_address 
  AND meter.meter_serial = '31602193' 
ORDER BY her060.report_time DESC 
LIMIT 10

结果很好：

meter_serial    node_address    report_time           node_reading
31602193    10164d995147    10/01/2014 0:36           777
31602193    10164d995147    8/01/2014 18:22           763
31602193    10164d995147    7/01/2014 6:04            724
31602193    10164d995147    6/01/2014 12:19           698
31602193    10164d995147    4/01/2014 23:55           669
31602193    10164d995147    3/01/2014 11:30           650
31602193    10164d995147    1/01/2014 23:06           624
31602193    10164d995147    31/12/2013 10:41          624
31602193    10164d995147    29/12/2013 22:17          624
31602193    10164d995147    28/12/2013 9:53           624

但我想添加另一列来计算node_readings之间的差异。

有人可以建议做什么吗？

另请原谅我的格式，这是我的第一个问题：）

提前感谢

Answer 1

SELECT 
  meter.meter_serial,
  her060.node_address,
  her060.report_time,
  her060.node_reading,
  @previous - node_reading AS your_difference, /*here the variable holds the value of the previous row*/
  @previous := node_reading  /*here the variable gets assigned the value of the current row*/
FROM
  meter
INNER JOIN her060 ON meter.node_address = her060.node_address 
CROSS JOIN (SELECT @previous := (SELECT node_reading FROM her060 ORDER BY report_time DESC LIMIT 1)) variable_initialization_query_alias
WHERE
   meter.meter_serial = '31602193' 
ORDER BY her060.report_time DESC 
LIMIT 10

Answer 2

对此列使用自动增量字段或唯一键，以便您可以减去上一行值。

**SELECT   meter.meter_serial,
  her060.node_address,
  her060.report_time,
  her060.node_reading ,
  a.node_reading -b.node_reading 
FROM
  meter,
  her060 a
 inner join 
her060 
 on a.id = b.id -1
WHERE meter.node_address = her060.node_address 
  AND meter.meter_serial = '31602193' 
ORDER BY her060.report_time DESC 
LIMIT 10**

结果MySQL的区别

2 个答案: