How to convert list of comma separated Ids into their name? - sql

Question

I have a table that contains:
id task_ids
1 10,15
2 NULL
3 17
I have the table that has the names of this tasks:
id task_name
10 a
15 b
17 c
I want to generate the following output
id task_ids task_names
1 10,15 a,b
2 null null
3 17 c
I know this structure isn't ideal but this is legacy table which I will not change now.
Is there easy way to get the output ?
I'm using Presto but I think this can be solved with native sql

Answer 1

WITH data AS (
SELECT * FROM (VALUES (1, '10,15'), (2, NULL)) x(id, task_ids)
),
task AS (
SELECT * FROM (VALUES ('10', 'a'), ('15', 'b')) x(id, task_name)
)
SELECT
d.id, d.task_ids
-- array_agg will obviously capture NULL task_name comping from LEFT JOIN, so we need to filter out such results
IF(array_agg(t.task_name) IS NOT DISTINCT FROM ARRAY[NULL], NULL, array_agg(t.task_name)) task_names
FROM data d
-- split task_ids by `,`, convert into numbers, UNNEST into separate rows
LEFT JOIN UNNEST (split(d.task_ids, ',')) AS e(task_id) ON true
-- LEFT JOIN with task to pull the task name
LEFT JOIN task t ON e.task_id = t.id
-- aggregate back
GROUP BY d.id, d.task_ids;

Answer 2

You have a horrible data model, but you can do what you want with a bit of effort. Arrays are better than strings, so I'll just use that:
select t.id, t.task_id, array_agg(tt.task_name) as task_names
from t left join lateral
unnest(split(t.task_ids, ',')) u(task_id)
on 1=1 left join
tasks tt
on tt.task_id = u.task_id
group by t.id, t.task_id;
I don't have Presto on hand to test this. But this or some minor variant should do what you want.
EDIT:
This version might work:
select t.id, t.task_id,
(select array_agg(tt.task_name)
from unnest(split(t.task_ids, ',')) u(task_id) join
tasks tt
on tt.task_id = u.task_id
) as task_names
from t ;