LEFT JOIN include data - sql

Question

I have an application which handles school vacation. Unfortunately there are three kinds of different school vacations: Country wide, Federal State wide and City wide vacations. I store all the information in a table days, a table vacation_periods and a connection table slots:
days {
id:integer
date_value:date
}
slots {
id:integer
day_id:integer
vacation_period_id:integer
}
vacation_periods {
id:integer
starts_on:date
ends_on:date
name:string
country_id:integer
federal_state_id:integer
city_id:integer
}
I want to select all days within a specific time frame. Let's say Jan 1st of 2017 to Jan 31st of 2017. I can get those days with:
SELECT * FROM days WHERE date_value >= '2017-01-01' AND
date_value <= '2017-01-31';
But for my vacation calendar I don't just need the days but also the information which vacation_periods are within. Assuming I search for all vacation_periods which are in that time frame and which have
country_id == 1 or federal_state_id == 5 or city_id == 30
I've read about JOINS and LEFT JOINS which seem to be the solution to the problem. But I can't get everything together.
Is it possible to send one SQL request which returns all days within the requested time frame and the additional information if a vacation_period that fits the country_id == 1 or federal_state_id == 5 or city_id == 30 rule is connected via slots to each day. Including the name of that vacation_period?
If one request is not possible: Which is the quickest way to solve this within the database? How many requests? What kind of requests?
If possible I'd like to get a result in some kind of this form:
- date_value: "2017-01-01"
- date_value: "2017-01-02"
- date_value: "2017-01-03"
* vacation_period.id: 15
* vacation_period.name: "foobar"
- date_value: "2017-01-04"
* vacation_period.id: 15
* vacation_period.name: "foobar"
- date_value: "2017-01-05"
* vacation_period.id: 15
* vacation_period.name: "foobar"
- date_value: "2017-01-06"
- date_value: "2017-01-07"
...

Answer 1

The following query might give you the answer you are looking for:
SELECT * FROM days WHERE date_value >= '2017-01-01' AND date_value <='2017-01-31'
INNER JOIN slots ON days.id = slots.day_id
INNER JOIN vacation_periods ON vacation_periods.id = slots.vacation_period_id

Answer 2

I think you can get an unformatted version of what you want (that could be processed into a hierarchical output) with
CREATE TYPE vacation_authority AS ENUM
('COUNTY', 'FED-STATE', 'CITY');
/* not necessary, but cleans up the vacation_period table */
change to let vacation_period have only one id, and a new field authority of type vacation_authority. You can now make a primary key out of either the id field or (id, authority), depending on how the vacation data comes into the system.
SELECT date_value, vp.name, vp.id /* is the ID meaningful or arbitrary? */
FROM dates LEFT JOIN vacation_periods vp
WHERE date_value BETWEEN vp.starts_on AND vp.ends_on; -- inclusive range
Now if there are multiple holidays spanning a given date, this will be multiple records in the output. It's not clear what you want in this case.

Answer 3

None of the other answers was able to solve my problem but they let me to the solution so I'm grateful for them. Here's the solution:
SELECT days.date_value, slots.period_id, vacation_periods.name FROM days
LEFT OUTER JOIN slots ON (days.id = slots.day_id)
LEFT OUTER JOIN vacation_periods ON (slots.period_id = vacation_periods.id)
WHERE days.date_value >= '2017-01-05'
AND days.date_value <='2017-01-15'
AND (vacation_periods.id IS NULL
OR vacation_periods.country_id = 1
OR vacation_periods.federal_state_id = 5)
ORDER BY days.date_value;