Group By Month And Show Total Days Remaining - mysql

Question

I want to list grouped by month and the days remaining to complete the course in the next column. The Course has 10 days.
Example data
ID Name Date
1 Sandy 2015-05-06
2 Candy 2015-05-06
3 Sandy 2015-05-28
4 Candy 2015-05-29
5 Candy 2015-06-01
Preferred output
| Name | Month | Attended | Remaining|
| Sandy| May | 2 | 8 |
| Candy| May | 2 | 8 |
| Candy| June | 1 | 7 |
If I use GROUP BY DATE_FORMAT(date, '%Y%m'), Name and try to do the calculation it does not work.

Answer 1

You need two different aggregates:
The number of days attended in the current month for a given user.
The number of days attended in all months up to and including the current month for a given user.
That's a tad fiddly, so 'tis time to do some Test-Driven Query Design (TDQD).
The table in the question is anonymous — that's such a common and irritating situation. So, the table is henceforth CourseAttendance, with the three columns shown in the data (ID, Name, Date).
Number of days attended by user in a specific month
Assuming the expression DATE_FORMAT(date, '%Y-%m') is syntactically valid, and that neither Date nor Month as a column name causes problems, then:
SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
Name,
COUNT(*) AS NumDays
FROM CourseAttendance
GROUP BY Month, Name
This should produce:
Month Name NumDays
2015-05 Sandy 2
2015-05 Candy 2
2015-06 Candy 1
Number of days attended by user up to and including a specific month
This time, the aggregate has to be over all dates less than or equal to the converted month value:
SELECT D.Month, D.Name, SUM(C.NumDays) AS TotDays
FROM (SELECT DISTINCT DATE_FORMAT(Date, '%Y-%m') AS Month, Name
FROM CourseAttendance
) AS D
JOIN (SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
Name,
COUNT(*) AS NumDays
FROM CourseAttendance
GROUP BY Month, Name
) AS C
ON C.Month <= D.Month AND C.Name = D.Name
GROUP BY D.Month, D.Name
This should give the output:
Month Name NumDays
2015-05 Sandy 2
2015-05 Candy 2
2015-06 Candy 3
Assembling the final result
The two previous result tables need to be joined on Month and Name, to yield the result:
SELECT A.Name, A.Month, A.NumDays AS Attended, (10 - B.TotDays) AS Remaining
FROM (SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
Name,
COUNT(*) AS NumDays
FROM CourseAttendance
GROUP BY Month, Name
) AS A
JOIN (SELECT D.Month, D.Name, SUM(C.NumDays) AS TotDays
FROM (SELECT DISTINCT DATE_FORMAT(Date, '%Y-%m') AS Month, Name
FROM CourseAttendance
) AS D
JOIN (SELECT DATE_FORMAT(Date, '%Y-%m') AS Month,
Name,
COUNT(*) AS NumDays
FROM CourseAttendance
GROUP BY Month, Name
) AS C
ON C.Month <= D.Month AND C.Name = D.Name
GROUP BY D.Month, D.Name
) AS B
ON A.Month = B.Month AND A.Name = B.Name
ORDER BY A.Name, A.Month
This should give output like:
Name Month Attended Remaining
Candy 2015-05 2 8
Candy 2015-06 1 7
Sandy 2015-05 2 8
You can fettle the month value into a month name if you need to. You can also fettle the sort order if you want month and name within month, etc.