MySQL Select use IN and GROUP BY - mysql

Question

I have two tables messages and users I want to find out which users received the messages however the query is only returning one message.
My Schemas are as follow
Messages
msg_id | msg_content | recipients |
-----------------------------------
1 | Hello world | 1,2,3,4,5
2 | Test | 1,3,5
3 | Welcome | 1,2,4
Users
uid | fname | lname |
---------------------------
1 | John |Doe |
2 | Jane |Doe |
3 | Mark |Someone |
4 | Mary |lady |
5 | Anthony |Doe |
So I would love to see my results simply as
msg_id | msg_content | recipients |
-----------------------------------
1 | Hello world | John,Jane,Mark,Mary,Anthony
2 | Test | John,Mark,Anthony
3 | Welcome | John,Jane,Mary
So I am doing my query as so
SELECT msg_id,msg_content,fname AS recepients FROM messages a
LEFT JOIN users ON uid IN(a.recipients)
When I run that query I only get one recipient. Please advice. Thanks.

Answer 1

I think you have to use a alternative way for create tables
Messages
msg_id | msg_content |
----------------------
1 | Hello world |
2 | Test |
3 | Welcome |
Users
uid | fname | lname |
---------------------------
1 | John |Doe |
2 | Jane |Doe |
3 | Mark |Someone |
4 | Mary |lady |
5 | Anthony |Doe |
users_has_messages
uhm_id | uid | msg_id |
---------------------------
1 | 1 | 1 |
2 | 2 | 1 |
3 | 3 | 1 |
4 | 2 | 2 |
5 | 1 | 3 |
Then you can use your code

Answer 2

Okay, so this schema isn't the best (using comma separated lists of IDs is not a great idea, and the performance of any joins will get pretty bad pretty quick). Best bet is to have a third table mapping uid's to msg_id's as mentioned by #Thilina.
That said, this query will do probably what you're after:
SELECT msg_id,msg_content,GROUP_CONCAT(fname) AS recepients FROM messages a
LEFT JOIN users ON FIND_IN_SET(uid, a.recipients)
GROUP BY msg_id

Answer 3

I tried this in Oracle 12c and it is working fine.
So basically what I did is
- Separate the userid from recipient field and used this a columns.
- Join with USERS table to get user fnames
- Used LISTAGG function to aggregate it back.
For MySql we need to find the corresponding functions to Separate the IDs between commas, Convert it to rows and Aggregate. But the inherent logic would be same.
with users (user_id,fname) as (
select 1 ,'John' from dual union
select 2 ,'Jane' from dual union
select 3 ,'Mark' from dual union
select 4 ,'Mary' from dual union
select 5 ,'Anthony' from dual
),
messages(msg_id, msg_content,recipients) as(
select 1,'Hello world','1,2,3,4,5' from dual union
select 2 , 'Test' ,'1,3,5' from dual union
select 3,' Welcome','1,2,4' from dual
),
flat as(
select msg_id,msg_content,
REGEXP_SUBSTR (recipients, '[^,]+', 1, COLUMN_VALUE) as user_id
from messages,
TABLE(
CAST(
MULTISET(
SELECT LEVEL
FROM DUAL
CONNECT BY LEVEL <= REGEXP_COUNT(recipients ,',' ) + 1
) AS SYS.ODCINUMBERLIST
)
)
),
unames as
( select f.msg_id,f.msg_content,u.fname from flat f inner join users u
on f.user_id = u.user_id
order by f.msg_id
)
SELECT msg_id,msg_content,LISTAGG(fname, ',') WITHIN GROUP (ORDER BY fname) as recipients
from unames
group by msg_id,msg_content