MySQL grouping by date range with multiple joins - mysql

Question

I currently have quite a messy query, which joins data from multiple tables involving two subqueries. I now have a requirement to group this data by DAY(), WEEK(), MONTH(), and QUARTER().
I have three tables: days, qos and employees. An employee is self-explanatory, a day is a summary of an employee's performance on a given day, and qos is a random quality inspection, which can be performed many times a day.
At the moment, I am selecting all employees, and LEFT JOINing day and qos, which works well. However, now, I need to group the data in order to breakdown a team or individual's performance over a date range.
Taking this data:
Employee
id | name
------------------
1 | Bob Smith
Day
id | employee_id | day_date | calls_taken
---------------------------------------------
1 | 1 | 2011-03-01 | 41
2 | 1 | 2011-03-02 | 24
3 | 1 | 2011-04-01 | 35
Qos
id | employee_id | qos_date | score
----------------------------------------
1 | 1 | 2011-03-03 | 85
2 | 1 | 2011-03-03 | 95
3 | 1 | 2011-04-01 | 91
If I were to start by grouping by DAY(), I would need to see the following results:
Day__date | Day__Employee__id | Day__calls | Day__qos_score
------------------------------------------------------------
2011-03-01 | 1 | 41 | NULL
2011-03-02 | 1 | 24 | NULL
2011-03-03 | 1 | NULL | 90
2011-04-01 | 1 | 35 | 91
As you see, Day__calls should be SUM(calls_taken) and Day__qos_score is AVG(score). I've tried using a similar method as above, but as the date isn't known until one of the tables has been joined, its only displaying a record where there's a day saved.
Is there any way of doing this, or am I going about things the wrong way?
Edit: As requested, here's what I've come up with so far. However, it only shows dates where there's a day.
SELECT COALESCE(`day`.day_date, qos.qos_date) AS Day__date,
employee.id AS Day__Employee__id,
`day`.calls_taken AS Day__Day__calls,
qos.score AS Day__Qos__score
FROM faults_employees `employee`
LEFT JOIN (SELECT `day`.employee_id AS employee_id,
SUM(`day`.calls_taken) AS `calls_in`,
FROM faults_days AS `day`
WHERE employee.id = 7
GROUP BY (`day`.day_date)
) AS `day`
ON `day`.employee_id = `employee`.id
LEFT JOIN (SELECT `qos`.employee_id AS employee_id,
AVG(qos.score) AS `score`
FROM faults_qos qos
WHERE employee.id = 7
GROUP BY (qos.qos_date)
) AS `qos`
ON `qos`.employee_id = `employee`.id AND `qos`.qos_date = `day`.day_date
WHERE employee.id = 7
GROUP BY Day__date
ORDER BY `day`.day_date ASC

Answer 1

The solution I'm comming up with looks like:
SELECT
`date`,
`employee_id`,
SUM(`union`.`calls_taken`) AS `calls_taken`,
AVG(`union`.`score`) AS `score`
FROM ( -- select from union table
(SELECT -- first select all calls taken, leaving qos_score null
`day`.`day_date` AS `date`,
`day`.`employee_id`,
`day`.`calls_taken`,
NULL AS `score`
FROM `employee`
LEFT JOIN
`day`
ON `day`.`employee_id` = `employee`.`id`
)
UNION -- union both tables
(
SELECT -- now select qos score, leaving calls taken null
`qos`.`qos_date` AS `date`,
`qos`.`employee_id`,
NULL AS `calls_taken`,
`qos`.`score`
FROM `employee`
LEFT JOIN
`qos`
ON `qos`.`employee_id` = `employee`.`id`
)
) `union`
GROUP BY `union`.`date` -- group union table by date
For the UNION to work, we have to set the qos_score field in the day table and the calls_taken field in the qos table to null. If we don't, both calls_taken and score would be selected into the same column by the UNION statement.
After this, I selected the required fields with the aggregation functions SUM() and AVG() from the union'd table, grouping by the date field in the union table.