how to update the table where results are coming from select statement

I would probably look at using the MERGE statement:
https://docs.oracle.com/en/database/oracle/oracle-database/12.2/sqlrf/MERGE.html#GUID-5692CCB7-24D9-4C0E-81A7-A22436DC968F

Specifically the WHEN MATCHED clause.  You will need to only use the inner select.  MERGE has problems when trying to UPDATE/INSERT into the same table that you select from.

There are many examples of MERGE out on the web.  I can't provide a copy/paste version because I still don't fully understand the requirements even though you have provided several samples of data and tables throughout all the previous related questions.

I am getting below error when i execute whole update statement.but when i execute only select statement its giving correct results.


or: ORA-01427: single-row subquery returns more than one row
01427. 00000 -  "single-row subquery returns more than one row"
*Cause:    
*Action:

<code>
UPDATE gt_add_isbns
   SET (error_message, FILENAME) =
          (SELECT CASE
                     WHEN category_exists = 0 THEN 'category not found'
                     WHEN final_documents IS NULL THEN 'documents not found'
                  END,
                  CASE
                     WHEN final_documents IS NOT NULL THEN final_documents
                  END
             FROM (SELECT category_exists, final_documents
                     FROM (
SELECT  (SELECT COUNT(*)
            FROM
            TEMP_NODE_PROPERTIES ANP,
    TEMP_NODE_PROPERTIES ANPP,
            TEMP_NODE AN
            WHERE
        ANP.NODE_ID=AN.ID AND
    ANPP.NODE_ID=ANP.NODE_ID AND
    AN.TYPE_QNAME='asset' AND
    ANP.STRING_VALUE ='uPDF' AND ANPP.STRING_VALUE ='Entire PDF'  AND
    ANP.TYPE_QNAME='categories'  AND
             ANP.NODE_ID IN(SELECT CHILD_ID
                  FROM TEMP_CHILD_ASSOC START WITH PARENT_ID IN(
                 SELECT CHILD_ID FROM TEMP_CHILD_ASSOC WHERE NAME in(GT.ISBN))
                      CONNECT BY PRIOR CHILD_ID = PARENT_ID)
    )  CATEGORY_EXISTS,
    (SELECT LISTAGG(T1.STRING_VALUE, ',') WITHIN GROUP  (ORDER BY T1.STRING_VALUE)
       FROM             TEMP_NODE_PROPERTIES T1
            WHERE   T1.TYPE_QNAME='name'  AND  (T1.STRING_VALUE LIKE '%pdf%'  AND  T1.STRING_VALUE NOT LIKE '%pod%')
         AND  T1.NODE_ID IN (SELECT ANP.NODE_ID
            FROM
            TEMP_NODE_PROPERTIES ANP,
    TEMP_NODE_PROPERTIES ANPP,
            TEMP_NODE AN
            WHERE
        ANP.NODE_ID=AN.ID AND
    ANPP.NODE_ID=ANP.NODE_ID AND
    AN.TYPE_QNAME='asset' AND
    ANP.STRING_VALUE ='uPDF' AND ANPP.STRING_VALUE ='Entire PDF' AND
    ANP.TYPE_QNAME='categories'  AND
             ANP.NODE_ID IN(SELECT CHILD_ID
                  FROM TEMP_CHILD_ASSOC START WITH PARENT_ID IN(
                 SELECT CHILD_ID FROM TEMP_CHILD_ASSOC WHERE NAME in(GT.ISBN))
                      CONNECT BY PRIOR CHILD_ID = PARENT_ID)))
     FINAL_DOCUMENTS
            FROM  GT_ADD_ISBNS GT)));
</code>