Use two aggregate functions in the same query

Question

Consider the following tables:

[Table: talks]
talkID | title        | starred
-------+--------------+--------
1      | talk1-title  | 1
2      | talk2-title  | 1
3      | talk3-title  | 0
4      | talk4-title  | 0
5      | talk5-title  | 0

[Table: talkspeaker]
talkID | speaker
-------+---------
1      | Speaker1
1      | Speaker2
2      | Speaker3
3      | Speaker4
3      | Speaker5
4      | Speaker6
5      | Speaker7
5      | Speaker8

[Table: similartalks]
talkID | similarTo
-------+----------
1      | 3
1      | 4
2      | 3
2      | 4
2      | 5
3      | 2
4      | 5
5      | 3
5      | 4

What I want to do is: Given the set of starred talks, I would like to select the top 2 of the unstarred talks (starred = 0) and their titles and speakers that are most similar to the set of starred talks. The problem is that getting the speakers requires using an aggregate function, and so does getting the most similar talks.

Without the speakers in the fray, I have been able to get the most similar talks using the following query:

select t2.talkID, t2.title, count(*) as count 
from similarTalks s, talks t1, talks t2
where s.talkID = t1.talkID
and t1.Starred = 1
and s.similarTo = t2.TalkID
and t2.Starred = 0
group by t2.title, t2.talkID
order by count desc
limit 2

Generally, I use the following aggregate function for getting the speakers, with appropriate group by columns (assume t = talkspeaker):

group_concat(t.speaker, ', ') as Speakers

as in

select t1.title, group_concat(t2.speaker, ', ') as Speakers 
from talks t1, talkspeaker t2
where t1.talkID = t2.talkID
group by t1.title

But I am not able to combine the two things together. It might matter that I am planning to run this query in a sqlite database (that is where the group_concat function comes from). The answer to the top 2 unstarred talks most similar to starred talks seem to be with talkIDs 3 and 4.

Answer 1

Firstly you might want to read this article about reasons to use ANSI 92 Joins instead of the aged ANSI 89 as used above. Secondly, SQLLite does support the GROUP_CONCAT function so you can use this.

You just neeed to add your second query as subquery into the first to get the desired result:

SELECT  Talks.TalkID, 
        Talks.Title, 
        ts.Speakers, 
        COUNT(*) AS SimilarTalks
FROM    Talks
        INNER JOIN SimilarTalks 
            ON Talks.TalkID = SimilarTalks.SimilarTo
        INNER JOIN Talks t2
            ON SimilarTalks.TalkID = t2.TalkID
            AND t2.Starred = 1
        INNER JOIN
        (   SELECT  TalkID, GROUP_CONCAT(Speaker, ',') AS Speakers
            FROM    TalkSpeaker
            GROUP BY TalkID
        ) ts
            ON ts.TalkID = Talks.TalkID
WHERE   Talks.Starred = 0
GROUP BY Talks.TalkID, Talks.Title, ts.Speakers
ORDER BY COUNT(*) DESC
LIMIT 2;

Example on SQL Fiddle

EDIT

You could also do this without a subquery using DISTINCT:

SELECT  Talks.TalkID, 
        Talks.Title, 
        GROUP_CONCAT(DISTINCT ts.Speaker) AS Speakers,
        COUNT(DISTINCT t2.TalkID) AS SimilarTalks
FROM    Talks
        INNER JOIN SimilarTalks 
            ON Talks.TalkID = SimilarTalks.SimilarTo
        INNER JOIN Talks t2
            ON SimilarTalks.TalkID = t2.TalkID
            AND t2.Starred = 1
        INNER JOIN TalkSpeaker ts
            ON ts.TalkID = Talks.TalkID
WHERE   Talks.Starred = 0
GROUP BY Talks.TalkID, Talks.Title
ORDER BY COUNT(DISTINCT t2.TalkID) DESC
LIMIT 2;

However I see no benefit at all in this method, and it is likely to be less efficient (I have not tested so can't be certain)

Answer 2

First, to get just the IDs of the desired talks, remove the other fields from your first query:

SELECT unstarred.talkID
FROM talks AS starred
  JOIN similarTalks AS s ON starred.talkID = s.talkID
  JOIN talks AS unstarred ON s.similarTo = unstarred.talkID
WHERE starred.starred
  AND NOT unstarred.starred
GROUP BY unstarred.talkID
ORDER BY COUNT(*) DESC
LIMIT 2

Then, use this as a subquery to get the information about the desired talks:

SELECT t.title AS Title,
       group_concat(s.speaker, ', ') AS Speakers
FROM talks AS t JOIN talkspeaker AS s ON t.talkID = s.talkID
WHERE t.talkID IN (SELECT unstarred.talkID
                   FROM talks AS starred
                     JOIN similarTalks AS s ON starred.talkID = s.talkID
                     JOIN talks AS unstarred ON s.similarTo = unstarred.talkID
                   WHERE starred.starred
                     AND NOT unstarred.starred
                   GROUP BY unstarred.talkID
                   ORDER BY COUNT(*) DESC
                   LIMIT 2)
GROUP BY t.talkID