Outer join with mandatory chain - sql

Question

Imagine I have this data
people -> address changes -> address change reasons
I want details of all people
I also want to know if they have ever changed address because of fire. So I don't want to know what the reasons are but if they have changed address for 1 single reason.
Each person could possibly have multiple address change reasons
so I have
SELECT people.*
CASE WHEN add_change.reason_id is not NULL THEN
'Y'
ELSE
'N'
END as been_fire
from people
left outer join add_change ON person.id = add_change.person
left outer join add_change_reason ON add_change.reason_id = add_change_reason.id AND add_change_reason.text = 'FIRE'
but this returns multiple rows per person if they have many address changes.
I can't just use
left outer join add_change ON add_change.person = person.id AND add_change.reason_id = 5
as this isn't fixed data.

Answer 1

Use exists:
SELECT p.*,
(CASE WHEN EXISTS (SELECT 1
FROM add_change ac JOIN
add_change_reason acr
ON ac.reason_id = acr.id
NVL(address_change.reason,'N')
WHERE p.id = ac.person AND
acr.text = 'FIRE'
)
THEN 'Y' ELSE 'N'
END) as has_fire_address_change
from people p;
Note that this changes the flag to 'Y' and 'N', which is what the description of your problem suggests that you want.

Answer 2

You could do a left join to a derived table that only returns person IDs that do have the change reason 'FIRE':
SELECT p.*
CASE WHEN cr.person IS NOT NULL THEN 'Y' ELSE 'N' END as been_fire
from people
left join (
select ac.person
from add_change ac
where exists (select *
from add_change_reason acr
where acr.id = ac.reason_id
AND acr.text = 'FIRE')
) cr on cr.person = p.id