Question

I have the following SQL and i want theorder_item_wo_id column to return null if the item does not exist in the nested sql and return the value of a column oi.cop_workorder_id if it does.

How can i accomplish this?

SELECT 
    cop.wo_id,
    CASE
     WHEN EXISTS (SELECT oi.cop_workorder_id 
            FROM oseo_orderitem  oi
            WHERE oi.cop_workorder_id = cop.wo_id) THEN 
            oi.cop_workorder_id
     ELSE null
    END AS order_item_wo_id
FROM oseo_cop_wo cop
where cop.wo_id = '123';

It looks like the oi.cop_workorder_id does not get recognized outside of the nested sql.

ERROR: missing FROM-clause entry for table “oi”

Answer 1

Unless I am missing something, it seems you are overcomplicating this.
Just use a LEFT JOIN:

SELECT 
    cop.wo_id,
    oi.cop_workorder_id AS order_item_wo_id
FROM oseo_cop_wo AS cop
    LEFT JOIN oseo_orderitem AS oi
        ON oi.cop_workorder_id = cop.wo_id
WHERE cop.wo_id = '123';