根据列中的值对内部联接求和

Question

I have a bookings table that is structured like this:

date        activity_id  state     pax  ...
----------  -----------  --------  ---
2018-01-01  1            accepted  2
2018-01-01  1            accepted  4
2018-01-01  1            pending   1
2018-01-01  2            accepted  3

I want to find the number of people that have an accepted or pending state per date and category. With the given rows, the result should be this:

date        activity_id  accepted  pending
----------  -----------  --------  ---
2018-01-01  1            6         1
2018-01-01  2            3         0

I don't care about other states, only accepted and pending.

To get only accepted or only pending bookings is simple enough:

SELECT date, activity_id, SUM(pax) AS accepted
FROM bookings
WHERE state = 'accepted'
GROUP BY date, activity_id

I tried to get both at the same time using something like this:

SELECT b1.date, b1.activity_id, SUM(b1.pax) AS accepted, SUM(b2.pax) AS pending
FROM bookings b1
JOIN bookings b2 ON b1.date = b2.date AND b1.activity_id = b2.activity_id
WHERE b1.state = 'accepted' AND b2.state = 'pending'
GROUP BY b1.date, b1.activity_id

but that only works for days when there are both accepted and pending bookings and the pending count seems to off sometimes.

Answer 1

You want conditional aggregation:

SELECT date, activity_id,
       SUM(case when state = 'accepted' then pax else 0 end) AS accepted,
       SUM(case when state = 'pending' then pax else 0 end) AS pending
FROM bookings
WHERE state in ('accepted', 'pending')
GROUP BY date, activity_id
ORDER BY date, activity_id;

Strictly speaking, the WHERE clause is not necessary. But if you have a lot of other states, then the filtering before the aggregation could benefit performance.