Fetch Unit consumption date-wise - mysql

Question

I am struggling in to get result from mysql in the following way. I have 10 records in mysql db table having date and unit fields. I need to get used units on every date.
Table structure as follows, adding today unit with past previous unit in every record:
Date Units
---------- ---------
10/10/2012 101
11/10/2012 111
12/10/2012 121
13/10/2012 140
14/10/2012 150
15/10/2012 155
16/10/2012 170
17/10/2012 180
18/10/2012 185
19/10/2012 200
Desired output will be :
Date Units
---------- ---------
10/10/2012 101
11/10/2012 10
12/10/2012 10
13/10/2012 19
14/10/2012 10
15/10/2012 5
16/10/2012 15
17/10/2012 10
18/10/2012 5
19/10/2012 15
Any help will be appreciated. Thanks

Answer 1

There's a couple of ways to get the resultset. If you can live with an extra column in the resultset, and the order of the columns, then something like this is a workable approach.
using user variables
SELECT d.Date
, IF(#prev_units IS NULL
,#diff := 0
,#diff := d.units - #prev_units
) AS `Units_used`
, #prev_units := d.units AS `Units`
FROM ( SELECT #prev_units := NULL ) i
JOIN (
SELECT t.Date, t.Units
FROM mytable t
ORDER BY t.Date, t.Units
) d
This returns the specified resultset, but it includes the Units column as well. It's possible to have that column filtered out, but it's more expensive, because of the way MySQL processes an inline view (MySQL calls it a "derived table")
To remove that extra column, you can wrap that in another query...
SELECT f.Date
, f.Units_used
FROM (
query from above goes here
) f
ORDER BY f.Date
but again, removing that column comes with the extra cost of materializing that result set a second time.
using a semi-join
If you are guaranteed to have a single row for each Date value, either stored as a DATE, or as a DATETIME with the timecomponent set to a constant, such as midnight, and no gaps in the Date value, and Date is defined as DATE or DATETIME datatype, then another query that will return the specifid result set:
SELECT t.Date
, t.Units - s.Units AS Units_Used
FROM mytable t
LEFT
JOIN mytable s
ON s.Date = t.Date + INTERVAL -1 DAY
ORDER BY t.Date
If there's a missing Date value (a gap) such that there is no matching previous row, then Units_used will have a NULL value.
using a correlated subquery
If you don't have a guarantee of no "missing dates", but you have a guarantee that there is no more than one row for a particular Date, then another approach (usually more expensive in terms of performance) is to use a correlated subquery:
SELECT t.Date
, ( t.Units - (SELECT s.Units
FROM mytable s
WHERE s.Date < t.Date
ORDER BY s.Date DESC
LIMIT 1)
) AS Units_used
FROM mytable t
ORDER BY t.Date, t.Units

Answer 2

spencer7593's solution will be faster, but you can also do something like this...
SELECT * FROM rolling;
+----+-------+
| id | units |
+----+-------+
| 1 | 101 |
| 2 | 111 |
| 3 | 121 |
| 4 | 140 |
| 5 | 150 |
| 6 | 155 |
| 7 | 170 |
| 8 | 180 |
| 9 | 185 |
| 10 | 200 |
+----+-------+
SELECT a.id,COALESCE(a.units - b.units,a.units) units
FROM
( SELECT x.*
, COUNT(*) rank
FROM rolling x
JOIN rolling y
ON y.id <= x.id
GROUP
BY x.id
) a
LEFT
JOIN
( SELECT x.*
, COUNT(*) rank
FROM rolling x
JOIN rolling y
ON y.id <= x.id
GROUP
BY x.id
) b
ON b.rank= a.rank -1;
+----+-------+
| id | units |
+----+-------+
| 1 | 101 |
| 2 | 10 |
| 3 | 10 |
| 4 | 19 |
| 5 | 10 |
| 6 | 5 |
| 7 | 15 |
| 8 | 10 |
| 9 | 5 |
| 10 | 15 |
+----+-------+

Answer 3

This should give the desired result. I don't know how your table is called so I named it "tbltest".
Naming a table date is generally a bad idea as it also refers to other things (functions, data types,...) so I renamed it "fdate". Using uppercase characters in field names or tablenames is also a bad idea as it makes your statements less database independent (some databases are case sensitive and some are not).
SELECT
A.fdate,
A.units - coalesce(B.units, 0) AS units
FROM
tbltest A left join tbltest B ON A.fdate = B.fdate + INTERVAL 1 DAY