Question

I have a table with columns id, user
I want to group by column id and show a list of users (comma separated) for each id.

In the final output I need to display:

user joe - id 1
users jim, mark, john - id 2
user dave - id 3
....

I’m getting error “Cardinality violation: 1242 Subquery returns more than 1 row” if I try this:

SELECT id, (SELECT distinct(user) FROM mytable b where a.id = b.id)
FROM mytable a
GROUP BY id

Answer 1

Since you are using MySQL, there is a built-in function for that which is the GROUP_CONCAT function. For example, you have a records like this:

ID      User
1       Joe
2       Jim
2       Mark
2       John
3       Dave

If you try to run this query:

SELECT ID, GROUP_CONCAT(User) Users
FROM tableName
GROUP BY ID

you will then have a result like this:

ID         Users
1          Joe
2          Jim, Mark, John
3          Dave