MySQL: why does GROUP_CONCAT lose rows in my multiple join query? - mysql

Question

I'm trying to fetch all the rows from table_m which also have an index in table_mi and I'm expecting to get 2 rows as a result (with m.id=3 and m.id=9) - but if I add GROUP_CONCAT to my select then I only get one row returned. Am I having a misshap somewhere within those joins of mine?
Query:
SELECT
m.id,
m.name,
m.keyword,
IFNULL(GROUP_CONCAT(r.keyword),'TEST') AS restrictions
FROM
table_m AS m
INNER JOIN
table_mi as mi ON m.id=mi.m_id
LEFT JOIN
table_ri as ri ON m.id=ri.m_id
LEFT JOIN
table_r AS r ON ri.r_id=r.id
WHERE
(
m.id>0
AND m.active=1
AND mi.p_id=0
AND (mi.pa_id="11" OR (mi.pa_id=0 AND mi.id!=0))
AND mi.u_id=IF((SELECT id FROM table_mi WHERE p_id=0 AND pa_id="11" AND u_id="2")>0,"2",0)
) OR mi.id=0
ORDER BY
mi.priority;
This is what I'm getting as a result:
ID NAME KEYWORD RESTRICTIONS
9 test_a key_a r_key_2,r_key_3,r_key_4
This is what I'm expecting:
ID NAME KEYWORD RESTRICTIONS
9 test_a key_a r_key_2,r_key_3,r_key_4
3 test_b key_b TEST
Please see my full example with schema on sql fiddle: http://sqlfiddle.com/#!2/359d9/1

Answer 1

GROUP_CONCAT is an aggregate function. It will bring back a single row UNLESS you specify a GROUP BY clause (with any fields that are not in the GROUP BY being aggregate fields)
Before the ORDER BY add the following:-
GROUP BY m.id, m.name, m.keyword
That said it looks like you might want to use CONCAT to join 2 values together rather than GROUP_CONCAT
As an aside, your SQL might be easier to read if you eliminate the subselect. Assuming it is bringing back a single record then possibly as follows
SELECT
m.id,
m.name,
m.keyword,
IFNULL(GROUP_CONCAT(r.keyword),'TEST') AS restrictions
FROM
table_m AS m
INNER JOIN
table_mi as mi ON m.id=mi.m_id
LEFT JOIN
table_ri as ri ON m.id=ri.m_id
LEFT JOIN
table_r AS r ON ri.r_id=r.id
LEFT OUTER JOIN
table_mi AS mi2 ON mi2.p_id=0 AND mi2.pa_id="11" AND mi2.u_id="2"
WHERE
(
m.id>0
AND m.active=1
AND mi.p_id=0
AND (mi.pa_id="11" OR (mi.pa_id=0 AND mi.id!=0))
AND mi.u_id=IF(mi2.id >0,"2",0)
) OR mi.id=0
ORDER BY
mi.priority;

Answer 2

You do no need GROUP_CONCAT to achieve what you want.
Instead of :
IFNULL(GROUP_CONCAT(r.keyword),'TEST') AS restrictions
use
IFNULL(r.keyword,'TEST') AS restrictions
OR:
Keep the query as it is and add GROUP BY m.id before ORDER BY