Using if statement in string_agg function - postreSQL - sql

Question

The query is as follows
WITH notes AS (
SELECT 891090 Order_ID, False customer_billing, false commander, true agent
UNION ALL
SELECT 891091, false, true, true
UNION ALL
SELECT 891091, true, false, false)
SELECT
n.order_id,
string_Agg(distinct CASE
WHEN n.customer_billing = TRUE THEN 'AR (Customer Billing)'
WHEN n.commander = TRUE THEN 'AP (Commander)'
WHEN n.agent = TRUE THEN 'AP (Agent)'
ELSE NULL
END,', ') AS finance
FROM notes n
WHERE
n.order_id = 891091 AND (n.customer_billing = TRUE or n.commander = TRUE or n.agent = TRUE)
GROUP BY ORDER_ID
As you can see there are two records with order_id as 891091.
First 891091 record has commander and agent set as true
Second 891091 record has customer_billing set as true
Since switch case is used, it considers only the first true value and returns commander and does not consider agent.
So the output becomes
order_id finance
891091 AP (Commander), AR (Customer Billing)
dbfiddle.uk Example
I need all the true values in the record to be considered so that the output becomes
order_id finance
891091 AP (Commander), AP (Agent), AR (Customer Billing)
My initial thought is that using if statement instead of case statement may fix this. I am not sure how to do this inside string_agg function
How to achieve this?
EDIT 1:
The answer specified below works almost fine. But the issue is that the comma separated values are not distinct
Here is the updated fiddle
https://dbfiddle.uk/?rdbms=postgres_14&fiddle=9647d92870e3944516172eda83a8ac6e

Answer 1

You can consider splitting your case into separate ones and using array to collect them. Then you can use array_to_string to format:
WITH notes AS (
SELECT 891090 Order_ID, False customer_billing, false commander, true agent UNION ALL
SELECT 891091, false, true, true UNION ALL
SELECT 891091, true, true, false),
tmp as (
SELECT
n.order_id id,
array_agg(
ARRAY[
CASE WHEN n.customer_billing = TRUE THEN 'AR (Customer Billing)' END,
CASE WHEN n.commander = TRUE THEN 'AP (Commander)' END,
CASE WHEN n.agent = TRUE THEN 'AP (Agent)' END
]) AS finance_array
FROM notes n
WHERE
n.order_id = 891091 AND (n.customer_billing = TRUE or n.commander = TRUE or n.agent = TRUE)
GROUP BY ORDER_ID )
select id, array_to_string(array(select distinct e from unnest(finance_array) as a(e)), ', ')
from tmp;
Here is db_fiddle.