How to get pairs from sqlite - sql

Question

I am a database and I am just playing with sqlite3 for a group project.
I have two tables that look something like this:
Table 1:
tv_show_id, tv_show_rating
Table 2:
tv_show_id, cast_id
Each tv_show has 1 unique id, but in table two there are multiple cast_ids for each tv_show
So we have something like this:
Table 1:
1234, 90
5678, 88
Table 2:
1234, "person 1"
1234, "person 2"
5678, "person 1"
5678, "person 3"
I want the following results: (person a, person b, # of shows together)
(person 1, person 2, 1)
(person 1, person 3, 1)
(person 2, person 1, 1)
(person 2, person 3, 0)
(person 3, person 1, 1)
(person 3, person 2, 0)
How can i use JOINS to get these results?

Answer 1

You can try this. Fiddle
select z.id,count(w.show_id) from
(
select distinct concat(x.cid,',',x.did) as id
from
(
select tt.cast_id as cid,ttt.cast_id as did
from t2 tt,t2 ttt
where tt.cast_id <> ttt.cast_id
) x
left join t2 on x.cid = t2.cast_id
) z
left join (select show_id, group_concat(cast_id order by cast_id) cid
from t2
group by show_id
union all
select show_id, group_concat(cast_id order by cast_id desc) cid
from t2
group by show_id
) w
on z.id = w.cid
group by z.id;

Answer 2

One way to do this (which might not be the most clever) would be to use a cross join to create a set of all possible pairs and the use a join and left join to determine if the pair have worked together.
In this query your source table is called table2:
select
cast1,
cast2,
sum(case when t2.cast_id is null then 0 else 1 end) as worked_together
from (
select distinct
t1.cast_id cast1,
t2.cast_id cast2
from table2 t1,table2 t2
where t1.cast_id != t2.cast_id
) p
join table2 t1 on p.cast1 = t1.cast_id
left join table2 t2 on t1.show_id = t2.show_id
and p.cast2 = t2.cast_id
and t1.cast_id != t2.cast_id
group by cast1, cast2;
Sample SQL Fiddle (tested with SQL.js). The query uses only ANSI SQL and is portable to other databases.
In some other databases you could have done this using a full outer join but SQLite does not support that construct.