SQL - Add query parameter with join - sql

Question

There're 3 tables. Is it possible by Persons.Id to get not only the city person lives in but also all the cities all her/his friends live in?
Eg for person with Id = 1 we get 2 items: Paris(he lives in) and New York(his friend Michele lives in)
Is it possible to make it one query using PostreSQL?
Persons
Id Name CityId
1 Mike 2
2 Bred 3
3 Michele 3
FriendsConnections
Id Person1 Person2
1 1 3
2 2 3
Cities
Id Title
1 London
2 Paris
3 New York

Answer 1

-- Get the city for Person 2
select c.Title
from Persons p
inner join Cities c on p.CityId = c.Id
where p.Id = 2
union
-- Get the friends cities for Person 2
select c.Title
from Persons p1
inner join FriendsConnections fc on p1.id = fc.Person1
inner join Persons p2 on fc.Person2 = p2.id
inner join Cities c on p2.CityId = c.Id
where p1.Id = 2

Answer 2

You can try this:
SELECT
Persons.Name AS Name,
Cities.Title AS City,
Friends.Name AS Friend,
FriendsCities.Title AS FriendsCity
FROM Persons
-- Join the friends
LEFT JOIN FriendsConnections
ON FriendsConnections.Person1 = Persons.Id
OR FriendsConnections.Person2 = Persons.Id
LEFT JOIN Persons AS Friends
ON (Friends.Id = FriendsConnections.Person1
OR Friends.Id = FriendsConnections.Person2)
AND Friends.Id != Persons.Id
-- Join the city
LEFT JOIN Cities
ON Cities.Id = Persons.CityId
-- Join the friends' cities
LEFT JOIN Cities AS FriendsCities
ON FriendsCities.Id = Friends.CityId
Maybe you prefer this one:
SELECT
Friend1.Name AS Friend1,
City1.Title AS City1,
Friend2.Name AS Friend2,
City2.Title AS City2
FROM FriendsConnections
-- Join the friends
LEFT JOIN Persons AS Friend1
ON Friend1.Id = FriendsConnections.Person1
LEFT JOIN Persons AS Friend2
ON Friend2.Id = FriendsConnections.Person2
-- Join the cities
LEFT JOIN Cities AS City1
ON City1.Id = Friend1.CityId
LEFT JOIN Cities AS City2
ON City2.Id = Friend2.CityId
You can maybe change the JOIN type, depending on the foreign keys you have.

Answer 3

So Michel is friend of Mike and Bred, and lives in New York, Mike lives in paris and Bred in New York.
I Believe you expect a result such as:
Name | CityName | FriendName | FriendCityName
Mike | Paris | Michel | New York
Bred | New York | Michel | New York
The SQL code valid for any relational database should be as follow:
SELECT p.name, pc.title as PersonCity, f.name as FriendName, fc.title as FriendCity
FROM Persons p
-- Getting all persons friends (one to many)
INNER JOIN FriendsConnections f
ON f.person1 = p.id
-- Getting all persons cities (one to one)
INNER JOIN Cities pc
ON pc.id = f.person1
-- Getting all friend cities (one to one)
INNER JOIN Cities fc
ON fc.id = f.person2
Hope this helps.

Answer 4

Since you mentioned to wanted to get a list of cities, the following query should work:
Demo on DB Fiddle
select c.title city
from persons p
inner join
(
select id, cityid
from persons
union
select pm.id, pf.cityid
from persons pm
inner join friendsconnections fc on fc.person1 = pm.id
inner join persons pf on pf.id = fc.person2
union
select pm.id, pf.cityid
from persons pm
inner join friendsconnections fc on fc.person2 = pm.id
inner join persons pf on pf.id = fc.person1
) p_to_c on p_to_c.id = p.id
left join cities c on c.id = p_to_c.cityid
where p.id = 1;
Result:
city
--------
New York
Paris

Answer 5

Following query will give you the expected output.
select name,city,friend,D.title as friendCity from
(select A.name as name,C.title as city,B.cityid as friendcityid,B.name as friend from persons A join
(select * from friendsconnections join persons on friendsconnections.person2=persons.id)
B on A.id=B.person1
join cities C on A.cityid=C.id
)tmp
join cities D on tmp.friendcityid=D.id
Output
name city friend friendCity
Mike Paris Michele New York
Bred New York Michele New York

Answer 6

I would recommend using exists or in for the matching. That is, the outer select is only on cities -- that way, you don't need to incur overhead to remove duplicates:
select c.*
from cities c
where exists (select p.cityId
from persons p
where p.id = 1 and c.id = p.cityId
) or
exists (select p.cityId
from persons p join
FriendsConnections fc
on p.id = fc.person2
where fc.person1 = 1 and c.id = p.cityId
);
Here is a db<>fiddle.