Cannot get the row that contain the last day of each month - sql

Question

I am quite new to SQL and I am trying to find the row that contains the last day of each month.
product_table example:
log_date | product_id | stock
10/30/2018 | 1001 | 59
10/29/2018 | 1002 | 100
10/28/2018 | 1003 | 2
...
9/30/2018 | 1001 | 1
9/30/2018 | 1002 | 45
This is my code:
SELECT *
FROM product_table
WHERE log_date IN
(
SELECT MAX(log_date)
FROM product_table
GROUP BY strftime('%m', log_date), strftime('%y', log_date)
)
Output:
9/9/2018 1001 28
9/9/2018 1002 94
9/9/2018 1003 29
9/9/2018 1004 89
9/9/2018 1005 3
9/9/2018 1006 46
...
Expected output:
9/30/2018 1001 28
9/30/2018 1002 94
9/30/2018 1003 29
...
8/31/2018 1001 89
8/31/2018 1002 3
...
7/31/2018 1001 46
...
I am working on a data file that date is a format like this: mm/dd/yyyy.
Should I change the date format to the normal way like yyyy-mm-dd because the code above returns the wrong result?
Do you guys know how to fix this? Thank you.

Answer 1

Update your table so the dates have the format YYYY-MM-DD which is the only valid date format for SQLite:
update product_table
set log_date =
substr(log_date, -4) || '-' ||
case
when log_date like '__/__/____' then
substr(log_date, 1, 2) || '-' || substr(log_date, 4, 2)
when log_date like '_/__/____' then
'0' || substr(log_date, 1, 1) || '-' || substr(log_date, 3, 2)
when log_date like '__/_/____' then
substr(log_date, 1, 2) || '-0' || substr(log_date, 4, 1)
when log_date like '_/_/____' then
'0' || substr(log_date, 1, 1) || '-0' || substr(log_date, 3, 1)
end;
Then your query should work.
This is a simplification of the GROUP BY clause:
SELECT *
FROM product_table
WHERE log_date IN
(
SELECT MAX(log_date)
FROM product_table
GROUP BY strftime('%Y%m', log_date)
)
See the demo.
Results:
| log_date | product_id | stock |
| ---------- | ---------- | ----- |
| 2018-10-30 | 1001 | 59 |
| 2018-09-30 | 1001 | 1 |
| 2018-09-30 | 1002 | 45 |

Answer 2

If you would like to get last day in a month based on log_date field, You have to use date function
Compute the last day of the current month.
SELECT date('now','start of month','+1 month','-1 day');
So, to get last day of current month, use:
SELECT T.log_date, date(substr(log_date, 6+T.noofdays, 4) || '-' || substr('00' || substr(log_date, 1, 1+T.noofdays), -2) || '-' || substr(log_date, 3+T.noofdays, 2), 'start of month', '1 month', '-1 day') AS lastdayofmonth
FROM (
SELECT log_date, instr(log_date, '/')-2 as noofdays
FROM product_table
) AS T;
DbFiddle
You can also use datetime function in the same way.
[EDIT]
Accorgingly to the discusssion in comment with #forpas (Thank you for your valuable comments)...
If 'last day of month' means 'last day of month existsing in a table', then MAX(log_date) should do the job. But if, you want to get 'last day of month' even if there's no corresponding date in a table, above query shows how to achieve that.
Good luck!