Need help for join and some calculations on a MySql insert - mysql

Question

I'll try to provide some context so you can understand what I'm trying to achieve here. My company uses open source software to manage the employees leaves (Jorani, feel free to google it :) ).
There are different types of leave (holidays, sick leave, etc.) and we want to calculate the days "not used" from the holidays of 2016 and "copy" them to another type of leave called "Remaining Holidays 2016".
The important tables are:
entitleddays (here you specify how many days of each type you give to an employee)
id employee startdate enddate type days description
661 3 2016-01-01 2017-02-28 1 14.00 Holidays 2016
1296 3 2016-01-01 2016-12-31 4 18.00 Sick leave 2016
leaves (this table has information about the leaves taken by the employees)
id startdate enddate status employee cause duration type
2436 2016-08-01 2016-08-01 3 78 OK from managers 1.00 1
2766 2016-09-05 2016-09-12 3 63 Holidays 6.00 1
So basically we have:
Entitled leaves:
Data stored in the entitleddays table shown above. In our example let's say I have 14 days for my 2016 holidays.
Taken leaves:
Leaves taken by the user, stored in the table called leaves shown above. For our example let's say I took a day off the first of August and 6 days on September.
Available leaves:
Available days are calculated: entitled days minus "taken leaves". For this examplee, 14 entitled days - 7 = 7 days. So I still have seven days available for holidays :D
So my goal is to insert these 7 days for this user as entitled days for the new type: "Remaining days from 2016" and do this for every user. So the solution that comes up to my mind is to do something like this for every user:
INSERT INTO entitleddays (employee, startdate, enddate, type, days, description)
SELECT id, '2017-01-01', '2017-02-31', '8', (entitled holidays for 2016 minus all the taken leaves of this type), 'Remaining holidays from 2016'
FROM users
Where 8 is the new type of leave where I want to copy the days (Remaining holidays from 2016).
For example I can get the taken holidays from 2016 for a specific user doing this:
SELECT SUM(duration)
FROM leaves
WHERE employee=3 AND status=3 AND type=1
Note: Type 1 is the type of leave "Holidays 2016" and status 3 means that the leave request was accepted.
I can probably achieve all of this in a single SQL instruction but it can also be split in more if simpler or easiest to manage/understand.
Many thanks in advance.

Answer 1

This is how you can handle the calculation:
sum the entitleddays in a subquery by grouping the datasets in its table per employee
maybe even group by year? In this case I just filtered for 2016 via WHERE-clause
sum the taken holidays in a subquery, again by grouping per employee
group by year or filter directly for the one you need
join this subquery onto the other resultset of the other query
calculate (entitled days - taken leaves) in the outer query
Query:
SELECT
entitled.employee,
'2017-01-01',
'2017-02-31',
'8' AS type,
entitled.days - takenDays.days,
'Remaining holidays from 2016'
FROM
(
SELECT
employee,
SUM(days) AS days
FROM
entitleddays
WHERE
startdate >= '2016-01-01'
AND type = 1
GROUP BY
employee
) AS entitled
LEFT JOIN (
SELECT
employee,
SUM(duration) AS days
FROM
`leaves`
WHERE
startdate >= '2016-01-01'
AND type = 1
GROUP BY
employee
) AS takenDays ON takenDays.employee = entitled.employee
I am not sure if this is how you want to calculate the sums for the days of entitleddays and taken days. The query just checks if startdate >= '2016-01-01'.
Also you mentioned a table users in your attempt but didn't provide details for the table, so I left it out. I guess you could use it as a basis otherwise. In the current query the grouped result of entitleddays is the basis.
For the insert
INSERT INTO entitleddays (employee, startdate, enddate, type, days, description)
SELECT
entitled.employee,
'2017-01-01',
'2017-02-31',
'8' AS type,
entitled.days - takenDays.days,
'Remaining holidays from 2016'
FROM
(
SELECT
employee,
SUM(days) AS days
FROM
entitleddays
WHERE
startdate >= '2016-01-01'
AND type = 1
GROUP BY
employee
) AS entitled
LEFT JOIN (
SELECT
employee,
SUM(duration) AS days
FROM
`leaves`
WHERE
startdate >= '2016-01-01'
AND type = 1
GROUP BY
employee
) AS takenDays ON takenDays.employee = entitled.employee