sqlite3 JOIN, GROUP_CONCAT using distinct with custom separator - sql

Question

Given a table of "events" where each event may be associated with zero or more "speakers" and zero or more "terms", those records associated with the events through join tables, I need to produce a table of all events with a column in each row which represents the list of "speaker_names" and "term_names" associated with each event.
However, when I run my query, I have duplication in the speaker_names and term_names values, since the join tables produce a row per association for each of the speakers and terms of the events:
1|Soccer|Bobby|Ball
2|Baseball|Bobby - Bobby - Bobby|Ball - Bat - Helmets
3|Football|Bobby - Jane - Bobby - Jane|Ball - Ball - Helmets - Helmets
The group_concat aggregate function has the ability to use 'distinct', which removes the duplication, though sadly it does not support that alongside the custom separator, which I really need. I am left with these results:
1|Soccer|Bobby|Ball
2|Baseball|Bobby|Ball,Bat,Helmets
3|Football|Bobby,Jane|Ball,Helmets
My question is this: Is there a way I can form the query or change the data structures in order to get my desired results?
Keep in mind this is a sqlite3 query I need, and I cannot add custom C aggregate functions, as this is for an Android deployment.
I have created a gist which makes it easy for you to test a possible solution: https://gist.github.com/4072840

Answer 1

Look up the speaker/term names independently from each other:
SELECT _id,
name,
(SELECT GROUP_CONCAT(name, ';')
FROM events_speakers
JOIN speakers
ON events_speakers.speaker_id = speakers._id
WHERE events_speakers.event_id = events._id
) AS speaker_names,
(SELECT GROUP_CONCAT(name, ';')
FROM events_terms
JOIN terms
ON events_terms.term_id = terms._id
WHERE events_terms.event_id = events._id
) AS term_names
FROM events

Answer 2

I ran accross this problem as well, but came up with a method that I found a bit easier to comprehend. Since SQLite reports SQLite3::SQLException: DISTINCT aggregates must have exactly one argument, the problem seems not so much related to the GROUP_CONCAT method, but with using DISTINCT within GROUP_CONCAT...
When you encapsulate the DISTINCT 'subquery' within a REPLACE method that actually does nothing you can have the relative simplicity of nawfal's suggestion without the drawback of only being able to concat comma-less strings properly.
SELECT events._id, events.name,
(group_concat(replace(distinct speakers.name),'',''), ' - ') AS speaker_names,
(group_concat(replace(distinct speakers.name),'',''), ' - ') AS term_names
FROM events
LEFT JOIN
(SELECT et.event_id, ts.name
FROM terms ts
JOIN events_terms et ON ts._id = et.term_id
) terms ON events._id = terms.event_id
LEFT JOIN
(SELECT sp._id, es.event_id, sp.name
FROM speakers sp
JOIN events_speakers es ON sp._id = es.speaker_id
) speakers ON events._id = speakers.event_id
GROUP BY events._id;
But actually I would consider this a SQLite bug / inconsistency, or am I missing something?

Answer 3

That's strange that SQLite doesnt support that!.
At the risk of being down voted, only if it helps:
You can avail Replace(X, Y, Z). But you have to be sure you wont have valid , values in your columns..
SELECT events._id, events.name,
REPLACE(group_concat(distinct speakers.name), ',', ' - ') AS speaker_names,
REPLACE(group_concat(distinct terms.name), ',', ' - ') AS term_names
FROM events
LEFT JOIN
(SELECT et.event_id, ts.name
FROM terms ts
JOIN events_terms et ON ts._id = et.term_id
) terms ON events._id = terms.event_id
LEFT JOIN
(SELECT sp._id, es.event_id, sp.name
FROM speakers sp
JOIN events_speakers es ON sp._id = es.speaker_id
) speakers ON events._id = speakers.event_id
GROUP BY events._id;

Answer 4

The problem arises only with the group_concat(X,Y) expression, not with the group_concat(X) expression.
group_concat(distinct X) works well.
So, if the ',' is good for you, there is no problem, but if you want a ';' instead of ',' (and you are sure no ',' is in your original text) you can do:
replace(group_concat(distinct X), ',', ';')

Answer 5

Just to put a proper workaround (murb's answer is strangely parenthesized).
problem:
group_concat(distinct column_name, 'custom_separator') takes custom_separator as a part of distinct.
solution:
We need some no-op to let SQLite know that distinct finished (to wrap distinct and it's arguments).
No-op can be replace with empty string as a second parameter (documentation to replace).
group_concat(replace(distinct column_name, '', ''), 'custom_separator')
edit:
just found that it does not work :-( - can be called but distinct is not working anymore

Answer 6

There is a special case that does not work in sqlite : group_concat(DISTINCT X, Y)
Whereas in SQL you can use group_concat(DISTINCT X SEPARATOR Y) in sqlite you can't
This example : Select group_concat(DISTINCT column1, '|') from example_table group by column2;
gives the result : DISTINCT aggregates must have exactly one argument At line 1:
The solution :
select rtrim(replace(group_concat(DISTINCT column1||'#!'), '#!,', '|'),'#!') from example_table