MYSQL Multiple Group Concat - mysql

Question

Hi I have a table called Engineers and a table called Post_Codes
When I use the following sql I get a list of engineers and the postcodes associated with them by using the Group Concat statement but I cannot figure out how to also include in another Group Concat (if indeed I need one) to also list in another field called Secondary_Post_Codes_Assigned those post codes linked to the same engineer via the Secondary_Engineer_id field.
SELECT
Engineer.Engineer,GROUP_CONCAT(Post_Code SEPARATOR ', ') as Post_Codes_Assigned,
Engineer.Region,
Engineer.active,
Engineer.Engineer_id
FROM Engineer INNER JOIN Post_Code ON Engineer.Engineer_id = Post_Code.Engineer_id
GROUP BY Engineer_id
What I need is output similar to this.
Engineer_id | Post_Codes_Assigned | Secondary_Post_Codes_Assigned
----------
1 | AW, AW3 | B12 |
2 | B12 | AW, CV12 |
I hope this is clear as I am pretty new to mysql.
Regards
Alan

Answer 1

You are already joining the primary post codes and list them, now do the same with the secondary ones.
SELECT
e.Engineer,
GROUP_CONCAT(DISTINCT pc1.Post_Code) AS Primary_Post_Codes_Assigned,
GROUP_CONCAT(DISTINCT pc2.Post_Code) AS Secondary_Post_Codes_Assigned,
e.Region,
e.active,
e.Engineer_id
FROM Engineer e
JOIN Post_Code pc1 ON e.Engineer_id = pc1.Engineer_id
JOIN Post_Code pc2 ON e.Engineer_id = pc2.Secondary_Engineer_id
GROUP BY e.Engineer_id;
As you see, you need DISTINCT because when selecting all primary and all secondary postcodes, you are getting rows for all combinations of them in the intermediate result. So you must get rid of duplicates. For this reason ist is better to aggregate before joining. (Which I generally consider a good idea, so you may want to make this a habit when working with aggregates.)
SELECT
e.Engineer,
pc1.Post_Codes AS Primary_Post_Codes_Assigned,
pc2.Post_Codes AS Secondary_Post_Codes_Assigned,
e.Region,
e.active,
e.Engineer_id
FROM Engineer e
JOIN
(
SELECT Engineer_id, GROUP_CONCAT(Post_Code) AS Post_Codes
FROM Post_Code
GROUP BY Engineer_id
) pc1 ON e.Engineer_id = pc1.Engineer_id
JOIN
(
SELECT Secondary_Engineer_id, GROUP_CONCAT(Post_Code) AS Post_Codes
FROM Post_Code
GROUP BY Secondary_Engineer_id
) pc2 ON e.Engineer_id = pc2.Secondary_Engineer_id;
A third option would be subqueries in the SELECT clause. I usually prefer them to be in the FROM clause as shown, because then it is easy to add more columns to the subqueries, which is not possible in the SELECT clause.
SELECT
e.Engineer,
(
SELECT GROUP_CONCAT(pc1.Post_Code)
FROM Post_Code pc1
WHERE pc1.Engineer_id = e.Engineer_id
) AS Primary_Post_Codes_Assigned,
(
SELECT GROUP_CONCAT(pc2.Post_Code)
FROM Post_Code pc2
WHERE pc2.Secondary_Engineer_id = e.Engineer_id
) AS Secondary_Post_Codes_Assigned,
e.Region,
e.active,
e.Engineer_id
FROM Engineer e;