mysql: Group Concat with joined tables and values that aren't unique - mysql

Question

I have an issue where I need to aggregate and concatenate multiple row data into single row output. I understand the tables are a problem in that their is no unique index, but I need to do this at the query level instead of the scripting level and I can't touch the database structure.
Here we go:
table: characteristics
id code_a code_b
-------------------------------
2201 CHAU AIRS
2201 CHAU PELC
2201 PROX AUTO
2201 PROX HOP`
table: characteristics_types
code description
-------------------------------
CHAU Heating System
PROX Nearby
table: characteristics_sub_types
code_a code description
-------------------------------
CHAU AIRS Forced Air
CHAU PELC Baseboard
PROX AUTO Highway
PROX HOP Hospital
Result required:
id Heating System Nearby
--------------------------------------------------------
2201 Forced Air, Baseboard Highway, Hospital
Not working:
SELECT id,
(case when C.code_a='CHAU' THEN GROUP_CONCAT(STC.description) ELSE NULL END) AS Heating System,
(case when C.code_a='PROX' THEN GROUP_CONCAT(STC.description) ELSE NULL END) AS Nearby
from characteristics C
inner join characteristics_types TC on C.code_a=TC.`code`
inner join characteristics_sub_types STC on C.code_a=STC.code_a and C.code_b=STC.`code`
GROUP BY C.id,C.code_a
I am getting the following results:
id Heating System Nearby
--------------------------------------------------------
2201 Forced Air, Baseboard NULL
2201 NULL Highway, Hospital
Any direction would be greatly appreciated!

Answer 1

GROUP_CONCAT accepts a DISTINCT keyword which is useful in fan-out-queries. You can also use ORDER BY within a GROUP_CONCAT if you need your results to be ordred. See Documentation. Using this, we can write your intended query like below:
SELECT
c.id,
GROUP_CONCAT(DISTINCT ct.description) as 'Heating System',
GROUP_CONCAT(DISTINCT cst.description) as 'Nearby'
FROM characteristics c
LEFT JOIN characteristics_types ct ON ct.id = c.id
LEFT JOIN characteristics_sub_types cst
ON cst ON cst.code_a = c.code_a AND cst.code = c.code_b
GROUP BY 1

Answer 2

I would write separate subqueries, each one preforming the group_concat that you need, and join them together. Individually, I wrote them like this:
SELECT c.id, GROUP_CONCAT(cst.description) AS 'Heating System'
FROM characteristics c
JOIN characteristics_sub_types cst ON cst.code_a = c.code_a AND cst.code = c.code_b AND c.code_a = 'CHAU'
GROUP BY c.id;
And the join like this:
SELECT c.id, t1.`Heating System`, t2.`Nearby`
FROM(
SELECT c.id, GROUP_CONCAT(cst.description) AS 'Heating System'
FROM characteristics c
JOIN characteristics_sub_types cst ON cst.code_a = c.code_a AND cst.code = c.code_b AND c.code_a = 'CHAU'
GROUP BY c.id) t1
JOIN(
SELECT c.id, GROUP_CONCAT(cst.description) AS 'Nearby'
FROM characteristics c
JOIN characteristics_sub_types cst ON cst.code_a = c.code_a AND cst.code = c.code_b AND c.code_a = 'PROX'
GROUP BY c.id) t2 ON t1.id = t2.id;
Here is a working SQL Fiddle.

Answer 3

You can try this way:
SELECT c.id,
GROUP_CONCAT(STC.description_heating) AS `Heating System`,
GROUP_CONCAT(STC.description_nearby) AS Nearby
from characteristics C
inner join characteristics_types TC on C.code_a=TC.`code`
inner join (
SELECT code_a, code,
CASE WHEN code_a = 'CHAU' THEN description ELSE null END as descriptio_heating,
CASE WHEN code_a = 'PROX' THEN description ELSE null END as descriptio_nearby,
FROM characteristics_sub_types
) as STC on C.code_a=STC.code_a and C.code_b=STC.`code`
GROUP BY C.id

Answer 4

SELECT c.id,
GROUP_CONCAT(STC.description_heating) AS `Heating System`,
GROUP_CONCAT(STC.description_nearby) AS Nearby
from characteristics C
inner join characteristics_types TC on C.code_a=TC.`code`
inner join (
SELECT code_a, code,
CASE WHEN code_a = 'CHAU' THEN description ELSE null END as description_heating,
CASE WHEN code_a = 'PROX' THEN description ELSE null END as description_nearby
FROM characteristics_sub_types
) as STC on C.code_a=STC.code_a and C.code_b=STC.`code`
GROUP BY C.id