0 - 3 of 3 tags for group_concat

I have a query that results in data containing customer id numbers, marketing channel, timestamp. So, the results might look something like this.

Hello Everyone!
I'm trying to aggregate all the field values of a particular group in one field.
For example, My input table looks like :-

FIELD1 GROUP1

A 1

A 2

B 1

B 2

C 1

C 3

3 null

 

and my output should look like :-

1 A,B,C

 

I want to aggregate as "comma-separated values" within a "group by", like MYSQL group_concat: