SQL 服务器 ISNULL function 在派生查询中不起作用

Question

I have the follow query that has a derived (sub-query) column in it. I am using the ISNULL function inside it, however my query output still has null values in it. Why is the ISNULL function not working as expected? The rows that you see resulting to NULL should actually be outputting as "Test"

SELECT 
    PS_PO_HDR.PO_ID,
    PS_PO_HDR.BUSINESS_UNIT,
    (SELECT ISNULL(PS_PO_COMMENTS_FS.COMMENTS_2000, 'TEST') 
     FROM PS_PO_COMMENTS_FS PS_PO_COMMENTS_FS  
     WHERE 
         -- PS_PO_LINE.PO_ID='0000000003' AND
         PS_PO_COMMENTS_FS.BUSINESS_UNIT = PS_PO_LINE.BUSINESS_UNIT 
         AND PS_PO_COMMENTS_FS.PO_ID = PS_PO_LINE.PO_ID 
         AND PS_PO_COMMENTS_FS.LINE_NBR = PS_PO_LINE.LINE_NBR 
         AND PS_PO_COMMENTS_FS.COMMENT_TYPE = 'LIN') AS "NOTE_TO_VENDOR",
    'DR' AS 'DISCOUNT_REASON'
FROM 
    PS_PO_LINE PS_PO_LINE, PS_PO_HDR PS_PO_HDR
WHERE 
    PS_PO_LINE.BUSINESS_UNIT = PS_PO_HDR.BUSINESS_UNIT
    AND PS_PO_LINE.PO_ID = PS_PO_HDR.PO_ID  
    AND PS_PO_HDR.PO_STATUS NOT IN ('C', 'X', 'PX')

Sample results:

PO_ID           BUSINESS_UNIT         NOTE_TO_VENDOR        DISCOUNT_REASON
---------------------------------------------------------------------------
0000000004      11MMS                 NULL                  DR
0000000016      41000                 NULL                  DR
0000000021      41000                 Test Note1            DR

Answer 1

The NULL is being returned by the subquery itself, not as a value from a row. The subquery returns no rows, and that is represented as NULL .

There are two options for handling this:

You can either use aggregation in the subquery:

(SELECT ISNULL(MAX(PS_PO_COMMENTS_FS.COMMENTS_2000), 'TEST') 
. . . 

The aggregation forces the subquery to return one row.

Or you can move the ISNULL() logic outside the subquery:

ISNULL( (SELECT PS_PO_COMMENTS_FS.COMMENTS_2000
         FROM . . .
        ), 'TEST'
      )

Answer 2

    SELECT 
PS_PO_HDR.PO_ID ,
PS_PO_HDR.BUSINESS_UNIT ,
**ISNULL(**(SELECT ISNULL(PS_PO_COMMENTS_FS.COMMENTS_2000, 'TEST') 
  FROM PS_PO_COMMENTS_FS PS_PO_COMMENTS_FS  
  WHERE -- PS_PO_LINE.PO_ID='0000000003' AND
  PS_PO_COMMENTS_FS.BUSINESS_UNIT=PS_PO_LINE.BUSINESS_UNIT 
  AND PS_PO_COMMENTS_FS.PO_ID=PS_PO_LINE.PO_ID 
  AND PS_PO_COMMENTS_FS.LINE_NBR=PS_PO_LINE.LINE_NBR 
  AND PS_PO_COMMENTS_FS.COMMENT_TYPE='LIN' ), **'TEST')** AS "NOTE_TO_VENDOR"  ,
'DR' AS 'DISCOUNT_REASON'
FROM PS_PO_LINE PS_PO_LINE, PS_PO_HDR PS_PO_HDR
WHERE 
 PS_PO_LINE.BUSINESS_UNIT=PS_PO_HDR.BUSINESS_UNIT
  AND PS_PO_LINE.PO_ID=PS_PO_HDR.PO_ID  
  AND PS_PO_HDR.PO_STATUS NOT IN ('C','X','PX')