PostgreSQL subquery split rows (error is more than one row returned by a subquery used as an expression) - postgresql

Question

this is my first post on stackoverflow so please be gentle. I have researched this problem and came up with many varied solutions...all of which seem to be just off from what I need. I have a postgresql subquery in a SELECT statement that returns multiple rows, which I know is obviously not ideal/allowed/sensible/etc.... However, this is the case for my current problem and I need to be able to take those multiple returned rows and apply them to every previous corresponding record row that they originally came out of.
Current Query:
SELECT cohead_number "Sales Order#",
cohead_custponumber "Purchase Order#",
item_number "Part Number",
item_descrip1 "Part Description",
CAST(shipitem_qty AS integer) "Item Quantity",
getPacklistItemLotSerial(shiphead_id, coitem_id) AS "LotNumbers" --this is the duplicating row subquery that I need listed in separate rows without changing other respective columns--
FROM cohead
LEFT JOIN coitem
ON coitem_cohead_id = cohead_id
LEFT JOIN shipitem
ON coitem_id = shipitem_orderitem_id
LEFT JOIN itemsite
ON coitem_itemsite_id = itemsite_id
LEFT JOIN item
ON itemsite_item_id = item_id
LEFT JOIN shiphead
ON shiphead_order_id = cohead_id
WHERE cohead_number = '79464' --this is just to test with one sales order instead of all (sales order being the input for the query)--
Results:
LINK: Results of above query here
What I Have Tried
Now, this line lets me split the column results via the delimiter ',' but I can't figure out how to get the results from this back into my original query's results:
(SELECT lot from regexp_split_to_table(getPacklistItemLotSerial(shiphead_id, coitem_id),', ') AS lot)
Results:
Here I input the shiphead_id and coitem_id for the example sales order so it can show you the resulting split column into rows.
SELECT lot from regexp_split_to_table(getPacklistItemLotSerial(22082, 50351),', ') AS lot
LINK: Results of Example
Please help walk me through what I need to do to achieve this. I imaging we need to declare some things and maybe join 2 tables in a more complex query...I don't really know. Thank you for any help you can offer.
EDIT
Added in the requested source code for the Function "getpacklistitemlotserial"
DECLARE
pShipheadId ALIAS FOR $1;
pOrderItemId ALIAS FOR $2;
_lotserial text;
_r RECORD;
_first BOOLEAN;
BEGIN
--Test to see if Lot/Serial Enabled
SELECT metric_value INTO _lotserial
FROM metric
WHERE ((metric_name='LotSerialControl')
AND (metric_value ='t'));
IF (FOUND) THEN
_lotserial := '';
_first := true;
FOR _r IN SELECT DISTINCT ls_number
FROM invdetail, invhist, shipitem, ls
WHERE ((shipitem_shiphead_id=pShipheadId)
AND (shipitem_orderitem_id=pOrderItemId)
AND (shipitem_invhist_id=invhist_id)
AND (invhist_id=invdetail_invhist_id)
AND (invdetail_ls_id=ls_id)) LOOP
IF (_first = false) THEN
_lotserial := _lotserial || ', ';
END IF;
_lotserial := _lotserial || _r.ls_number;
_first := false;
END LOOP;
RETURN _lotserial;
ELSE
RETURN '';
END IF;
END

Answer 1

Try:
SELECT DISTINCT cohead_number "Sales Order#",
cohead_custponumber "Purchase Order#",
item_number "Part Number",
item_descrip1 "Part Description",
CAST(shipitem_qty AS integer) "Item Quantity",
-- getPacklistItemLotSerial(shiphead.shiphead_id, coitem.coitem_id) AS "LotNumbers" --this is the duplicating row subquery that I need listed in separate rows without changing other respective columns--
CASE WHEN EXISTS(
SELECT metric_value FROM metric
WHERE metric_name='LotSerialControl' AND metric_value ='t'
)
THEN x.ls_number
ELSE ''
END AS "LotNumbers"
FROM cohead
LEFT JOIN coitem
ON coitem_cohead_id = cohead_id
LEFT JOIN shipitem
ON coitem_id = shipitem_orderitem_id
LEFT JOIN itemsite
ON coitem_itemsite_id = itemsite_id
LEFT JOIN item
ON itemsite_item_id = item_id
LEFT JOIN shiphead
ON shiphead_order_id = cohead_id
LEFT JOIN (
SELECT ls_number,
shipitem_shiphead_id, -- parameter: pShipheadId
shipitem_orderitem_id -- parameter pOrderItemId
FROM invdetail, invhist, shipitem, ls
WHERE
-- (shipitem_shiphead_id=pShipheadId)
-- AND (shipitem_orderitem_id=pOrderItemId)
(shipitem_invhist_id=invhist_id)
AND (invhist_id=invdetail_invhist_id)
AND (invdetail_ls_id=ls_id)
) x
ON ( x.shipitem_shiphead_id = shiphead.shiphead_id
AND
x.shipitem_orderitem_id = coitem.coitem_id
)
WHERE cohead_number = '79464'