mysql query "GROUP BY" giving random results - mysql

Question

I was here once before and was assisted tremendously:
mysql group by returning incorrect result
I am now stuck with a similar Query which has an added table:
here is the Fiddle http://sqlfiddle.com/#!2/7fe40/5
There are 3 tables: Project, Task and Timesheet
The Task table has a foreign key (project_id) which links back to the Parent Project, it also holds a value for "assigned hours".
The timesheet table holds the "actual_hours" and the users name.
what I am trying to do is SUM how many actual hours have been spent on each project.
this query returns incorrect and random results:
SELECT DISTINCT
`timesheet`.`name`,
SUM(`timesheet`.`hours`) AS `total_hours`,
`project`.`project_name`,
`task`.`task_name`,
SUM(`task`.`hrs`) AS `assigned_hours`
FROM
`task`
INNER JOIN `project` ON (`task`.`project_id` = `project`.`project_id`)
INNER JOIN `timesheet` ON (`task`.`task_id` = `timesheet`.`task_id`)
GROUP BY
`project`.`project_name`
Any help greatly appreciated.
Thanks

Answer 1

Mysql has support for non-standard grouping. The fix is to group by all non-aggregate selected columns:
...
GROUP BY `timesheet`.`name`, `project`.`project`, `task`.`task_name`
You should also remove the DISTINCT keyword.
In mysql only (all other databases will raise an exception), grouping by fewer than all non-aggregated columns results in a random row for each unique combination of those columns that are grouped by (although in practice, it isn't a random row but the first row encountered - YMMV is you rely on this).

Answer 2

If you just want the sum for the projects, why are you including the tasks?
SELECT SUM(ts.`hours`) AS `total_hours`,
p.`project_name`,
SUM(t.`hrs`) AS `assigned_hours`
FROM `task` t INNER JOIN
`project` p
ON t.`project_id` = p.`project_id`) INNER JOIN
`timesheet` ts
ON t.`task_id` = ts.`task_id`
GROUP BY p.`project_name`;
I have a feeling though that this might give inaccurate results because there may be multiple timesheet records for each task. So you can try:
SELECT SUM(ts.total_hours) AS `total_hours`,
p.`project_name`,
SUM(t.`hrs`) AS `assigned_hours`
FROM `task` t INNER JOIN
`project` p
ON t.`project_id` = p.`project_id` INNER JOIN
(select ts.task_id, sum(ts.hours) as total_hours
from `timesheet` ts
group by ts.task_id
) ts
ON t.`task_id` = ts.`task_id`
GROUP BY p.`project_name`;
Here is a SQL Fiddle.