How to subtract two columns in different table - sql

Question

I have a table of ward
ward_number | class | capacity
________________________________________
1 | A1 | 1
2 | A1 | 2
3 | B1 | 3
4 | C | 4
5 | B2 | 5
capacity = how many beds there is in the ward
I also have a table called ward_stay:
ward_number | from_date | to_date
_____________________________________________
2 | 2015-01-01 | 2015-03-08
3 | 2015-01-16 | 2015-02-18
6 | 2015-03-05 | 2015-03-18
3 | 2015-04-15 | 2015-04-20
1 | 2015-05-19 | 2015-05-30
I want to count the number of beds available in ward with class 'B1' on date '2015-04-15':
ward_number | count
_____________________
3 | 2
How to get the count is basically capacity - the number of times ward_number 3 appears
I managed to get the number of times ward_number 3 appears but I don't know how to subtract capacity from this result.
Here's my code:
select count(ward_number) AS 'result'
from ward_stay
where ward_number = (select ward_number
from ward
where class = 'B1');
How do I subtract capacity from this result?

Answer 1

SQL Fiddle Demo
Using 2015-01-17 instead I calculate the total of occupied bed on that day. Then join back to substract from original capacity. in case all bed are free the LEFT JOIN will return NULL, so COALESCE will put 0
SELECT w."ward_number", "capacity" - COALESCE(occupied, 0) as "count"
FROM wards w
LEFT JOIN (
SELECT "ward_number", COUNT(*) occupied
FROM ward_stay
WHERE to_date('2015-01-17', 'yyyy-mm-dd') BETWEEN "from_date" and "to_date"
GROUP BY "ward_number"
) o
ON w."ward_number" = o."ward_number"
WHERE w."class" = 'B1'
OUTPUT
| ward_number | count |
|-------------|-------|
| 3 | 2 |

Answer 2

select w.ward_number,
w.capacity - count(ws.ward_number) AS "result"
from ward as w left join ward_stay as ws
on ws.ward_number = w.ward_number
and date '2015-05-19' between ws.from_date and ws.to_date
where w.class = 'B1' -- which class
-- bed not occupied on that date
group by w.ward_number, w.capacity
having w.capacity - count(*) > 0 -- only available wards
See fiddle

Answer 3

You need to aggregate both tables before returning, because you have multiple rows for the same word type in both. So:
select c.class, (c.capacity - coalesce(wc.occupied)) as available
from (select class, sum(capacity) as capacity
from ward
group by class
) c left join
(select w.class, count(*) as occupied
from ward_stay ws join
ward s
on ws.ward_number = w.ward_number and
'2015-05-19' between ws.from_date and ws.to_date
) wc
on w.class = wc.class;
Note: this is standard SQL except for the date constant. This works in most databases; some might have other formats (or it might depend on internationalization settings).
Strictly speaking the aggregation on ward is not necessary for "B1". But it is clearly necessary for "A1".