So if I had a database where there was a table person like so:
Person:
id: (unique)
name
And a second table that created relationships between rows like so:
Friend:
person1 (foreign key to Person.id)
person2 (foreign key to Person.id)
How would I query friends for a person like so:
select (other person) from Friend where person1=(me) or person2=(me);
You could use a CASE clause. For example, if you're looking for all persons who are friends of person with id = 5...
SELECT CASE
WHEN person1 = 5 THEN person2 ELSE person1 END AS friendId
FROM Friend
WHERE (person1 = 5) OR (person2 = 5)
Naturally you'll use a variable where I'm hard-coding a value of 5.
I understand that person2 and person1 are friends
select a.person2 from
(
select person1, person2 from Friend union select person2, person1 from Friend
)
a inner join Person b where b.id = a.person1
where b.name = 'nameofPersonne'
or
where b.id = 5
If you like to select your friends write this following request:
select isnull(f1.person2, f2.person1)
from person p
left join friend f1 on p.id = f1.person1
left join friend f2 on p.id = f2.person2
join friend f on p.id = f.person1 or p.id = f.person2
where p.id = you
In Postgres, you can use a lateral join:
select v.otherperson
from friend f cross join lateral
(values (person1, person2), (person2, person1)) v(me, otherperson)
where v.me = 5;
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