How to spot missing value from two GROUP_CONCAT columns? - mysql

Question

I have table All Badges and table User Badge.
The value in User Badge can all be found on the All Badges and each user_id can have multiple User Badge so I use GROUP_CONCAT to group the value based on the user_id. The goal of the User Badge is to allow users to collect all available badges, thus they will need information about the missing badge on their profile.
Here is the All Badges table:
|---------------|-----------|
| Badge_ID | Label |
|---------------|-----------|
| 1 | a |
| 2 | b |
| 3 | c |
| 4 | d |
| 5 | e |
|---------------|-----------|
The User Badge table:
|------------------|---------------|---------------|---------------|
| User_Badge_ID | Badge_Label | user_ID | fk_badge_id |
|------------------|---------------|---------------|---------------|
| 1 | a | 1 | 1 |
| 2 | a | 2 | 1 |
| 3 | b | 1 | 2 |
| 4 | a | 3 | 1 |
| 5 | b | 2 | 2 |
| 6 | c | 2 | 3 |
| 7 | d | 2 | 4 |
|------------------|---------------|---------------|---------------|
I have this query to group the badge on user_id, but I don't know the query to show the missing value.
SELECT
user_badge.user_id AS User_ID
GROUP_CONCAT(user_badge.Badge_Label) AS User_Badge
FROM
User_Badge
The desired result. The missing badge will be added into the new column called Missing Badge.
|---------------------|------------------|------------------|
| User_ID | User_Badge | Missing Badge |
|---------------------|------------------|------------------|
| 1 | a, b | c, d, e |
|---------------------|------------------|------------------|
| 2 | a, b, c, d | e |
|---------------------|------------------|------------------|
| 3 | a | b, c, d, e |
|---------------------|------------------|------------------|
Any ideas to show the missing value from each user_id? Many thanks for your willingness to help.

Answer 1

select s.user_id,
group_concat(case when fk_badge_id is not null then s.label end) 'user_badge',
group_concat(case when fk_badge_id is null then s.label end) 'missing'
from
(
SELECT distinct USER_ID,badge_id,ab.label
FROM USER_BADGE ub
CROSS JOIN ALL_BADGES ab
) s
left join user_badge ub on ub.user_id = s.user_id and ub.fk_badge_id = s.badge_id
group by s.user_id;
Where the sub query s gets all the badges a user could have using a cross join and the main query checks those he/she has or not , the left join returning nulls if has not.
+---------+------------+---------+
| user_id | user_badge | missing |
+---------+------------+---------+
| 1 | a,b | c,d,e |
| 2 | a,b,c,d | e |
| 3 | a | c,d,e,b |
+---------+------------+---------+
3 rows in set (0.00 sec)