Group Concat in Redshift - sql

Question

I have a table like this:
| Col1 | Col2 |
|:-----------|------------:|
| 1 | a;b; |
| 1 | b;c; |
| 2 | c;d; |
| 2 | d;e; |
I want the result to be some thing like this.
| Col1 | Col2 |
|:-----------|------------:|
| 1 | a;b;c;|
| 2 | c;d;e;|
Is there some way to write a set function which adds unique values in a column into an array and then displays them. I am using the Redshift Database which mostly uses postgresql with the following difference:
Unsupported PostgreSQL Functions

Answer 1

Have a look at Redshift's listagg() function which is similar to MySQL's group_concat. You would need to split the items first and then use listagg() to give you a list of values. Do take note, though, that, as the documentation states:
LISTAGG does not support DISTINCT expressions
(Edit: As of 11th October 2018, DISTINCT is now supported. See the docs.)
So will have to take care of that yourself. Assuming you have the following table set up:
create table _test (col1 int, col2 varchar(10));
insert into _test values (1, 'a;b;'), (1, 'b;c;'), (2, 'c;d;'), (2, 'd;e;');
Fixed number of items in Col2
Perform as many split_part() operations as there are items in Col2:
select
col1
, listagg(col2, ';') within group (order by col2)
from (
select col1, split_part(col2, ';', 1) as col2 from _test
union select col1, split_part(col2, ';', 2) as col2 from _test
)
group by col1
;
Varying number of items in Col2
You would need a helper here. If there are more rows in the table than items in Col2, a workaround with row_number() could work (but is expensive for large tables):
with _helper as (
select
(row_number() over())::int as part_number
from
_test
),
_values as (
select distinct
col1
, split_part(col2, ';', part_number) as col2
from
_test, _helper
where
length(split_part(col2, ';', part_number)) > 0
)
select
col1
, listagg(col2, ';') within group (order by col2) as col2
from
_values
group by
col1
;