How to concatenate values into string in MySQL - mysql

Question

I have this SQL table:
user_skills
==========================
user_id | skill_id | value
1 | 4 | 1
1 | 5 | 1
1 | 6 | 1
1 | 7 | 1
1 | 8 | 1
2 | 4 | 1
2 | 6 | 1
4 | 4 | 1
4 | 5 | 1
5 | 8 | 1
Than, I have SQL query which returns me info about user.
$mysqli->query("SELECT u.*,
COUNT(a.user_id) AS jobs,
r.*
FROM users u
LEFT JOIN articles a
ON u.id = a.user_id
LEFT JOIN rating r
ON r.user_id = u.id
LEFT JOIN regions r1
ON r1.id = u.city
WHERE u.active = 1 AND
u.server_id IN (" . $server_id . ")
GROUP BY (CASE WHEN a.user_id IS NULL THEN u.id ELSE a.user_id END)
ORDER BY u.points DESC,
jobs_done DESC,
u.id DESC
LIMIT " . (($page - 1) * $limit) . ", " . ($page * $limit));
Now, I need to select to each user their skills, or better, skill_ids. Ideally as a string separated by commas because we spoke about only 5 skill_id (4-8).
I tried:
$mysqli->query("SELECT u.*,
COUNT(a.user_id) AS jobs,
r.*,
CONCAT_WS(',', us.skill_id) AS skill_ids
FROM users u
LEFT JOIN articles a
ON u.id = a.user_id
LEFT JOIN rating r
ON r.user_id = u.id
LEFT JOIN regions r1
ON r1.id = u.city
LEFT JOIN user_skills us
ON us.user_id = u.id AND skill_id IN (4, 5, 6, 7, 8)
WHERE u.active = 1 AND
u.server_id IN (" . $server_id . ")
GROUP BY (CASE WHEN a.user_id IS NULL THEN u.id ELSE a.user_id END)
ORDER BY u.points DESC,
jobs_done DESC,
u.id DESC
LIMIT " . (($page - 1) * $limit) . ", " . ($page * $limit));
But it returns me only the first skill_id from database, without concatenating.
When I tried to use CONCAT_WS(',', us.skill_id) AS skill_ids, result was comma and the first one skill_id (, 4).
Data in database are okay, SUM(us.skill_id) returns 30 for user with id=4.
Any idea?

Answer 1

You have two major problems...
A) The function you want is GROUP_CONCAT(), which produces a CSV of values for the group. This query give you the skills list for each user:
select user_id, group_concat(skill_id) as skill_ids
from user_skills
group by user_id
B) A more major problem is your incorrect use of GROUP BY: For it to work as expected, you must list every column that is not an aggregate function, either using positional numbers or expressions.
Assuming users table has 6 columns and rating has 4:
SELECT
u.*,
COUNT(a.user_id) AS jobs,
r.*,
GROUP_CONCAT(us.skill_id) AS skill_ids
FROM ...
...
GROUP BY 1,2,3,4,5,6, 8,9,10,11,12 -- note 7 is missing
or you can list column expressions in the GROUP BY:
GROUP BY u.id, u.name, ..., r.id, r.foo, ...
If you omit even one non- aggregate column, it won't work as expected. This peculiar behaviour is mysql only - every other database throws a syntax exception if you left one out.