group by year on multiple date columns mysql - mysql

Question

I have table as following:
hours | ... | task_assigned | task_deadline | task_completion
----------------------------------------------------------------
123 | ... | 2019-08-01 | - | -
234 | ... | - | 2018-08-01 | 2019-08-01
145 | ... | 2017-08-01 | 2017-08-01 | 2018-01-01
I want to calculate total hours for each year, i.e. grouping by year.
Currently I'm only taking into account task_completion field.
If there's no value in task_completion field, the record is not included in SUM calculation.
To elaborate further, say for year 2019, row 1 and 1 both should be considered. Hence the total hours should be 123 + 234 = 357.
And for year 2018, row 2 and 3.
Similarly, for year 2017, row 3.
SELECT YEAR(task_completion) as year, ROUND(SUM(total_hours), 2) as hours
FROM task
GROUP BY year
HAVING year BETWEEN '$year_from' AND '$year_to'
The resultset:
year | hours
--------------------
2017 | <somevalue>
2018 | <somevalue>
2019 | <somevalue>
How can I include other two date fields too?

Answer 1

You want to consider each row once for each of its years. Use UNION to get these years:
select year, round(sum(total_hours), 2) as hours
from
(
select year(task_assigned) as year, total_hours from task
union
select year(task_deadline) as year, total_hours from task
union
select year(task_completion) as year, total_hours from task
) years_and_hours
group by year
having year between $year_from and $year_to
order by year;
If you want to consider a row with one year twice or thrice also as often in the sum, then change UNION to UNION ALL.

Answer 2

Basically, you want to unpivot the data. I will assume that the - represents a NULL value and your dates are real dates.
select year(dte) as year, sum(total_hours) as hours
from ((select task_assigned as dte, total_hours
from task
) union all
(select task_deadline, total_hours
from task
) union all
(select task_completion, total_hours
from task
)
) d
where dte is not null
group by year(dte)
order by year(dte);
Based on your sample data, the round() is not necessary so I removed it.
If you want to filter for particular years, the filtering should be in a where clause -- so it filters the data before aggregation.
Change the where to:
where year(dte) >= ? and year(dte) <= ?
or:
where dte >= ? and dte <= ?
to pass in the dates.
The ? are for parameter placeholders. Learn how to use parameters rather than munging query strings.

Answer 3

This answer is no langer valid with the updated request.
If I understand correctly, you want to use task_assigned if the task_completion is still null. Use COALEASCE for this.
SELECT
YEAR(COALESCE(task_completion, task_assigned)) as year,
ROUND(SUM(total_hours), 2) as hours
FROM task
GROUP BY year
HAVING year BETWEEN $year_from AND $year_to
ORDER BY year;
(I don't think you actually want to use task_deadline, too, for how could a task get completed before getting assigned first? If such can occur, then include it in the COALESCE expression. Probably: COALESCE(task_completion, task_assigned, task_deadline)` then.)