MySQL query to see changes during the month of last 3 month - mysql

Question

I want to see changes during the month of last 3 month.
Database:
MySQL:
SELECT DATE_FORMAT(CUR.time, '%Y %b') AS thisTime,
MAX(CUR.value) AS thisValue,
MAX(PRE.value) AS prevValue,
MAX(CUR.value)-MAX(PRE.value) AS compValue
FROM test CUR
INNER JOIN test PRE
ON MONTH(PRE.time) = (SELECT MAX(MONTH(XXX.time))
FROM test XXX
WHERE MONTH(XXX.time) < MONTH(CUR.time))
WHERE CUR.time > DATE_SUB(NOW(), INTERVAL 3 MONTH)
GROUP BY MONTH(CUR.time)
ORDER BY CUR.id
Result:
Can see that there is no January!
I want to see changes during the month of last 3 month like this.
Anyone got ideas?
| 2019 Feb | 50 | 40 | 10 |
| 2019 Jan | 40 | 25 | 15 |
| 2018 Dec | 25 | 20 | 5 |

Answer 1

The problem with trying to use MONTH in a JOIN condition is the wrap around from 12 to 1 at the end of a year. It's easiest just to use the date as a YEARMONTH string:
SELECT DATE_FORMAT(t1.time, '%b %Y') AS thisTime,
MAX(t1.value) AS thisValue,
MAX(t2.value) AS prevValue,
MAX(t1.value) - MAX(t2.value) AS compValue
FROM test t1
JOIN test t2 ON DATE_FORMAT(t2.time, '%Y%m') = (SELECT MAX(DATE_FORMAT(`time`, '%Y%m'))
FROM test t3
WHERE DATE_FORMAT(t3.time, '%Y%m') < DATE_FORMAT(t1.time, '%Y%m'))
GROUP BY thisTime
ORDER BY STR_TO_DATE(CONCAT('1 ', thisTime), '%d %b %Y') DESC
Output:
thisTime thisValue prevValue compValue
Feb 2019 50 40 10
Jan 2019 40 25 15
Dec 2018 25 20 5
Demo on dbfiddle

Answer 2

Instead of trying a complicated join, you can just use sub-queries since the table isn't very complicated. Also just filter for the results on the 2nd of every month since you don't need the 1st from what your expected result shows.
SELECT
DATE_FORMAT(cur.time,'%Y %b') AS thisTime,
cur.value AS thisValue,
(SELECT value FROM test WHERE time = DATE_SUB(cur.time,INTERVAL 1 MONTH)) AS prevValue,
(SELECT cur.value - value FROM test WHERE time = DATE_SUB(cur.time,INTERVAL 1 MONTH)) AS compValue
FROM
test AS cur
WHERE
cur.time >= DATE_SUB(CURDATE(),INTERVAL 3 MONTH)
AND DATE_FORMAT(cur.time,'%d') = 2
Result:
thisTime thisValue prevValue compValue
2019 Feb 50 40 10
2019 Jan 40 25 15
2018 Dec 25 20 5
EDIT
Per Nick's point below, I did assume that the max would always be the 2nd of every month. If that's false and the table snapshot just doesn't happen to show a month where the 1st was the max, then this modification will give the correct max result.
The JOIN of monthMax and the regular test table will guarantee you always pull the max value per month, no matter how many dates there are within that month. Then just use a similar sub-query method as my initial query.
SELECT
DATE_FORMAT(t.time,'%Y %b') AS thisTime,
t.value AS thisValue,
(SELECT MAX(value) FROM test WHERE DATE_FORMAT(time,'%Y-%m') = DATE_FORMAT(DATE_SUB(t.time,INTERVAL 1 MONTH),'%Y-%m')) AS prevValue,
(SELECT t.value - MAX(value) FROM test WHERE DATE_FORMAT(time,'%Y-%m') = DATE_FORMAT(DATE_SUB(t.time,INTERVAL 1 MONTH),'%Y-%m')) AS compValue
FROM
(SELECT DATE_FORMAT(time,'%Y-%m') AS 'timeFix', MAX(value) AS 'maxValue' FROM test GROUP BY timeFix) AS monthMax
JOIN (test AS t) ON (monthMax.maxValue = t.value AND monthMax.timeFix = DATE_FORMAT(t.time,'%Y-%m'))
WHERE
t.time > DATE_SUB(CURDATE(),INTERVAL 3 MONTH)
Result:
thisTime thisValue prevValue compValue
2019 Feb 50 40 10
2019 Jan 40 25 15
2018 Dec 25 20 5