[英]mysql subtract two rows from column and places into an alias
I have table meter_readings
with columns: id
, date_taken
, kwh
. 我的表meter_readings
包含列: id
, date_taken
, kwh
。
I'm trying to subtract two rows in kwh
column together and put the results into an alias called consumption
. 我正在尝试将kwh
列中的两行相减,然后将结果放入一个名为consumption
的别名中。
I'm using: 我正在使用:
SELECT id, kwh COALESCE(kwh-(SELECT kwh FROM meter_readings WHERE id= id+1), kwh) AS consumption
FROM meter_readings;
What I get back in consumption alias is simple the same as the original kwh
: 我所得到的消耗别名很简单,与原始kwh
相同:
id date_taken kwh consumption
1 2013-01-01 4567.89 4567.89
2 2013-01-08 4596.71 4596.71
3 2013-01-15 4607.89 4607.89
what I would like is: 我想要的是:
id date_taken kwh consumption
1 2013-01-01 4567.89 0
2 2013-01-08 4596.71 28.11
3 2013-01-15 4607.89 11.18
so id 1 = 0 because this is the first date_taken kwh reading so has no need for a consumption value. 所以id 1 = 0,因为这是第一个date_taken kwh读数,因此不需要消耗值。 This is trying to calculate over a year the weekly kwh consumption. 这试图计算一年中每周的千瓦时消耗量。
Just give the table name an alias, and the table inside the correlated subquery a different alias name. 只需为表名指定一个别名,然后在相关子查询中为表指定一个不同的别名即可。 Something like this: 像这样:
SELECT
m1.id,
m1.kwh,
COALESCE(m1.kwh - (SELECT m2.kwh
FROM meter_readings AS m2
WHERE m2.id = m1.id + 1),
m1.kwh) AS consumption
FROM meter_readings AS m1;
This will give you: 这将为您提供:
| ID | KWH | CONSUMPTION |
------------------------------
| 1 | 4567.89 | 1141.18 |
| 2 | 3426.71 | 1181.37 |
| 3 | 2245.34 | 2245.34 |
For the updated sample data, just use WHERE m2.id = m1.id - 1
inside the correlated subquery with COALESCE(..., 0)
so that the first one will be 0. Like this: 对于更新的样本数据,只需在具有COALESCE(..., 0)
的相关子查询中使用WHERE m2.id = m1.id - 1
COALESCE(..., 0)
以便第一个为0。像这样:
SELECT
m1.id,
date_format(m1.date_taken, '%Y-%m-%d') AS date_taken,
m1.kwh,
COALESCE(m1.kwh - (SELECT m2.kwh
FROM meter_readings m2
WHERE m2.id = m1.id - 1), 0) AS consumption
FROM meter_readings m1;
This will give you: 这将为您提供:
| ID | DATE_TAKEN | KWH | CONSUMPTION |
-------------------------------------------
| 1 | 2013-01-01 | 4567.89 | 0 |
| 2 | 2013-01-08 | 4596.71 | 28.82 |
| 3 | 2013-01-15 | 4607.89 | 11.18 |
COALESCE返回第一个非null值,因此您必须在不使用COALESCE的情况下尝试执行此查询,然后看看问题出在哪里
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.