Oracle Statement does not count correctly - sql

Question

I've got a SQL-statement with a - for me not explainable - strange behaviour.
Perhaps you could find what's wrong:
When I use the statement
select count(*) from department
I got 2755 results
Using the following statement
select
building1.street, building1.streetno, building1.plz, building1.city, dept1.buildingid
from
department dept1
left join
supporter sup
on
dept.supporterid = sup.id
left join
building building1
on
sup.buildingid = building1.ibuildingid
where
dept.usepostaladresssupporter = 1
union all
select
building2.street, building2.streetno, building2.plz, building2.city, dept2.buildingid
from
building building2
right join
tueks_department dept2
on
dept2.buildingid = building2.ibuildingid
where
dept2.usepostaladresssupporter = 0
I got 2755 results too.
But when I want to combine the two statements with a left join:
select count(*) from department
left join
(
select
building1.street, building1.streetno, building1.plz, building1.city, dept1.buildingid
from
department dept1
left join
supporter sup
on
dept.supporterid = sup.id
left join
building building1
on
sup.buildingid = building1.ibuildingid
where
dept.usepostaladresssupporter = 1
union all
select
building2.street, building2.streetno, building2.plz, building2.city, dept2.buildingid
from
building building2
right join
tueks_department dept2
on
dept2.buildingid = building2.ibuildingid
where
dept2.usepostaladresssupporter = 0
) postadress
on
department.buildingid = postadress.buildingid;
I got 3648513 results.
My expectation was, that I get only 2755 results.
Where's the mistake?
Thanks for help!

Answer 1

I assume that buildingid is not unique (for my reasoning to hold true, it can't be unique)
Imagine following simple tables
TableA
create TableA (name VARCHAR(32));
insert into TableA values ('Lieven');
insert into TableA values ('Lieven');
TableB
create TableB (name VARCHAR(32));
insert into TableB values ('Lieven');
insert into TableB values ('Lieven');
insert into TableB values ('AnyOtherValue');
Select statement
select * from TableA a left outer join TableB b on a.name = b.name
As each record of TableA is matched with each record of TableB where the name is equal, this will result in 4 records (the AnyOtherValue is dissmissed as it doesn't match)
The first record of TableA is returned with two of three records of `TableB'
The second record of TableA is returned with two of three records of `TableB'

Answer 2

The query
select
building1.street, building1.streetno, building1.plz, building1.city, dept1.buildingid
from
department dept1
left join
supporter sup
on
dept.supporterid = sup.id
left join
building building1
on
sup.buildingid = building1.ibuildingid
where
dept.usepostaladresssupporter = 1
union all
select
building2.street, building2.streetno, building2.plz, building2.city, dept2.buildingid
from
building building2
right join
tueks_department dept2
on
dept2.buildingid = building2.ibuildingid
where
dept2.usepostaladresssupporter = 0
will return one row per department that has usepostaladresssupporter as either 0 or 1 (note that records with other values will not be included, this may or may not be a problem depending on the constrainst of this column).
The unique key of this query results is probably something like departmentid (you will need to include that column in your select criteria).
So the correct query should look something like this:
select * from department
left join
(
select
building1.street, building1.streetno, building1.plz, building1.city, dept1.departmentid
from
department dept1
left join
supporter sup
on
dept.supporterid = sup.id
left join
building building1
on
sup.buildingid = building1.ibuildingid
where
dept.usepostaladresssupporter = 1
union all
select
building2.street, building2.streetno, building2.plz, building2.city, dept2.departmentid
from
building building2
right join
tueks_department dept2
on
dept2.buildingid = building2.ibuildingid
where
dept2.usepostaladresssupporter = 0
) postadress
on
department.departmentid = postadress.departmentid;
Your query will go wrong on data something like this:
Departmentid BuildingId Name
1 1 Dept1
2 2 Dept2
3 2 Dept3
The multiplying effect is not quite equal to deptcount * deptcount, but rather it is buildingcount * buildingcount + deptcount - buildingcount