Aggregate result of sub query to comma separated values - postgresql

Question

How to get get the result of subquery as comma separated values.
I have three tables , location and stock_location_type and location_label.
I am joining location and stock_location_type and based on the result of SLT.inventory_location_cd , I am querying another table location_label.
To do that I am writing following query.
select L.stock_catalogue_id, SLT.inventory_location_cd,
case
when nventory_location_cd = 'base location' then (select related_location_id from location_label where base_location_id = location_id)
when nventory_location_cd != 'base location' then (select base_location_id from location_label where related_location_id = location_id)
end as "Current Location",
* from location L
join stock_location_type SLT on L.stock_location_type_id = SLT.stock_location_type_id;
These subquueries returns multiple rows.
I tried using string_agg and casting related_location_id and base_location_id (as they are UUIDs). But then it complains about group by.
If I use group by then it errors out , 'multiple rows returned by subquery'.
What am I missing?

Answer 1

I recreated your set of tables with
A location_label table
create table location_label(location_id int, location_label varchar);
insert into location_label values(1, 'Home');
insert into location_label values(2, 'Office');
insert into location_label values(3, 'Garage');
insert into location_label values(4, 'Bar');
A stock_location_type table
create table stock_location_type (stock_location_type_id int, inventory_location_cd varchar);
insert into stock_location_type values(1, 'base location');
insert into stock_location_type values(2, 'Not base location');
A location table
create table location (stock_catalogue_id int, base_location_id int, related_location_id int, stock_location_type_id int);
insert into location values(1,1,2,1);
insert into location values(1,2,1,2);
insert into location values(1,3,3,1);
insert into location values(2,4,3,1);
insert into location values(2,3,1,1);
insert into location values(2,2,4,2);
If I understand your statement correctly you are trying to join location and location_label tables based on the inventory_location_cd column using either base_location_id or location_id.
If this is what you're trying to achieve, the following query should do it. By moving the join condition in the proper place
select L.stock_catalogue_id,
SLT.inventory_location_cd,
location_id "Current Location Id",
location_label "Current Location Name"
from location L join stock_location_type SLT
on L.stock_location_type_id = SLT.stock_location_type_id
left outer join location_label
on (
case when
inventory_location_cd = 'base location'
then base_location_id
else related_location_id
end) = location_id
;
result is
stock_catalogue_id | inventory_location_cd | Current Location Id | Current Location Name
--------------------+-----------------------+---------------------+-----------------------
1 | base location | 1 | Home
1 | Not base location | 1 | Home
1 | base location | 3 | Garage
2 | base location | 3 | Garage
2 | base location | 4 | Bar
2 | Not base location | 4 | Bar
(6 rows)
if you need to aggregate it up by stock_catalogue_id and inventory_location_cd, that can be achieved with
select L.stock_catalogue_id,
SLT.inventory_location_cd,
string_agg(location_id::text, ',') "Current Location Id",
string_agg(location_label::text, ',') "Current Location Name"
from location L join stock_location_type SLT
on L.stock_location_type_id = SLT.stock_location_type_id
left outer join location_label
on (case when inventory_location_cd = 'base location' then base_location_id else related_location_id end) = location_id
group by L.stock_catalogue_id,
SLT.inventory_location_cd;
with the result being
stock_catalogue_id | inventory_location_cd | Current Location Id | Current Location Name
--------------------+-----------------------+---------------------+-----------------------
1 | base location | 1,3 | Home,Garage
1 | Not base location | 1 | Home
2 | base location | 3,4 | Garage,Bar
2 | Not base location | 4 | Bar
(4 rows)

Answer 2

You can use the string_agg function to aggregate the values in a comma-separated string. So your sub-queries needs to be rewritten to
select string_agg(related_location_id, ', ') from location_label where base_location_id = location_id
and
select string_agg(base_location_id, ', ') from location_label where related_location_id = location_id