I have table meter_readings
with columns: id
, date_taken
, kwh
.
I'm trying to subtract two rows in kwh
column together and put the results into an alias called consumption
.
I'm using:
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
:
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:
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. This is trying to calculate over a year the weekly kwh consumption.
Just give the table name an alias, and the table inside the correlated subquery a different alias name. Something like this:
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:
| 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:
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:
| 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
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