SELECT CASE WHEN not working?

Question

Hello, can't figure out why the following statement continues to return NULL values and seems to not recognize my case when portion?...
INSERT tbl_Main
select OrderID
 ,(select
  case
   when (b.OrderValue is null or b.OrderValue = '') then 'Y'
   else b.OrderValue
  end
 from tbl_OrderDetails b where b.OrderID = a.OrderID and b.DetailID = '7')
AS OrderStatus
from tbl_Order a
Thanks in advanced.

Answer 1

Please post DDL, so that people do not have to guess what the keys, constraints, Declarative Referential Integrity, data types, etc. in your schema are. Learn how to follow ISO-11179 data element naming conventions and formatting rules. Temporal data should
use ISO-8601 formats. Code should be in Standard SQL as much as possible and not local dialect. 
This is minimal polite behavior on SQL forums. 
1. The use of the “tbl-” prefix is a design flaw called “tibbling” and people will laugh at you for it. 
2. Do not nest selects inside a select list; the performance stinks. 
3. Why do you think “A”, "D” etc are good aliases that will make maintaining this code easier? Noobs do this is match the names of tape drives on old hardware! 
4. Why do you think that treating a NULL and an empty string as the same is a good idea? Use a CHECK (order_value <> ''). 
My guess is you could write this instead: 
INSERT INTO Orders
SELECT order_id, COALESCE (D.order_value, 'Y') AS order_status
 FROM Order_Details AS D 
WHERE D.order_id = Orders.order_id 
  AND D.detail_id = '7')
AS order_status; 
But this still smells bad. A status is a state of being so it has a temporal component. Order details do not usually have an id; the item identifier (EAN, UPC, SKU, or whatever) is used. 
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