Alias scoping in SQL - sql

Question

I'm having an issue with a complex query on an SQLite3 database that I think has to do with a misunderstanding on my part of how to refer to columns in a results table returned by a select statement, especially when aliases are involved.
Here is an example table - a list of movie IDs with a row for each actor working on the movie:
CREATE TABLE movie_actor (imdb_id TEXT, actor TEXT);
INSERT INTO movie_actor VALUES('44r4', 'John Doe');
INSERT INTO movie_actor VALUES('44r4', 'Jane Doe');
INSERT INTO movie_actor VALUES('44r4', 'Jermaine Doe');
INSERT INTO movie_actor VALUES('44r4', 'Jacob Doe');
INSERT INTO movie_actor VALUES('55r5', 'John Doe');
INSERT INTO movie_actor VALUES('55r5', 'Jane Doe');
INSERT INTO movie_actor VALUES('55r5', 'Nathan Deer');
INSERT INTO movie_actor VALUES('66r6', 'Bob Duck');
INSERT INTO movie_actor VALUES('66r6', 'John Doe');
INSERT INTO movie_actor VALUES('66r6', 'Jermaine Doe');
INSERT INTO movie_actor VALUES('66r6', 'Jane Doe');
INSERT INTO movie_actor VALUES('77r7', 'John Doe');
I am trying to find out the how many times each pair of actors worked with each other across all movies. I decided to go about this with a self-join, but ran into issues where I would get record pairs such as "John Doe, Jane Doe, 3" and "Jane Doe, John Doe, 3" - this is really the same thing, and I wanted to only count the first version. This is the code that resulted:
SELECT DISTINCT
CASE WHEN d.actor_1 > d.actor_2 THEN d.actor_1 ELSE d.actor_2 END d.actor_1,
CASE WHEN d.actor_2 > d.actor_1 THEN d.actor_2 ELSE d.actor_1 END d.actor_2,
d.v
FROM (
SELECT c.actor_1 AS actor_1, c.actor_2 AS actor_2, COUNT(*) AS v
FROM (
SELECT a.actor AS actor_1, b.actor AS actor_2
FROM movie_actor a JOIN movie_actor b ON a.imdb_id=b.imdb_id
) AS c
WHERE c.actor_1 <> c.actor_2
GROUP BY c.actor_1, c.actor_2
HAVING COUNT(*) > 2
ORDER BY COUNT(*) DESC
LIMIT 20
)
AS d
This doesn't run, but I can't figure out why. My assumption is that I am not using aliases properly, but I really don't know. Any ideas?
(SQL Fiddle link here)

Answer 1

We get a simpler query, if we add the condition a.actor < b.actor. This excludes pairs with equal actors and at the same time it removed the need of swapping actors.
SELECT
a.actor AS actor_1, b.actor AS actor_2, COUNT(*) AS v
FROM
movie_actor a
INNER JOIN movie_actor b
ON a.imdb_id = b.imdb_id
WHERE
a.actor < b.actor
GROUP BY a.actor, b.actor
ORDER BY COUNT(*) DESC, a.actor, b.actor
LIMIT 20
Note: SQL always creates a cross product when joining, i.e. it creates all possible combinations of records that match the join condition. Therefore for imdb 55r5 (including 3 actors) it will first generate the following 3 x 3 = 9 pairs:
John Doe John Doe
John Doe Jane Doe
John Doe Nathan Deer
Jane Doe John Doe
Jane Doe Jane Doe
Jane Doe Nathan Deer
Nathan Deer John Doe
Nathan Deer Jane Doe
Nathan Deer Nathan Deer
Then the WHERE-clause excludes all a >= b pairs and we get
John Doe Nathan Deer
Jane Doe John Doe
Jane Doe Nathan Deer

Answer 2

Generate the distinct pairs first, then count them.
select actor_1, actor_2, count(*)
from (select distinct a.imdb_id, a.actor as actor_1, b.actor as actor_2
from movie_actor a
inner join movie_actor b on a.imdb_id = b.imdb_id
where a.actor < b.actor) x
group by actor_1, actor_2
order by actor_1, actor_2;
actor_1 actor_2 count(*)
---------- ---------- ----------
Bob Duck Jane Doe 1
Bob Duck Jermaine D 1
Bob Duck John Doe 1
Jacob Doe Jane Doe 1
Jacob Doe Jermaine D 1
Jacob Doe John Doe 1
Jane Doe Jermaine D 2
Jane Doe John Doe 3
Jane Doe Nathan Dee 1
Jermaine D John Doe 2
John Doe Nathan Dee 1