Use NVL, GROUP BY, HAVING COUNT to check if record are same

I have 2 tables: CUST_DETAILS with column CUST_REF_ID and CUST_MERGE with columns NEW_CUST_REF_ID and OLD_CUST_REF_ID, below script will check if both columns in CUST_MERGE having same country id with the corresponding column in CUST_DETAILS, and return Y, else return N

SELECT
   NVL
      ((SELECT 'Y' FROM DUAL WHERE EXISITS
       (
        SELECT CTRY_ID FROM CUST_DETAILS cust_dtl
        INNER JOIN CUST_MERGE cust_merge
        ON cust_dtl.CUST_REF_ID=cust_merge.NEW_CUST_REF_ID
        AND cust_dtl.CUST_REF_ID=cust_merge.OLD_CUST_REF_ID
        GROUP BY CTRY_ID
        HAVING COUNT(CTRY_ID)>1
        )),'N') AS SAME_CTRY_ID
FROM DUAL;

The data in both tables are all having same country_id, meaning expected result is 'Y'. But 'N' returned for now, unless if I change condition from AND to OR, then it work as what expected,even the subquery also return with correct country ID

Sample Data

TABLE CUST_MERGE

 NEW_CUST_REF_ID                        OLD_CUST_REF_ID

 B5000                                   B6000

 B5000                                   A6000

TABLE CUST_DETAILS

 CUST_REF_ID                CTRY_ID  
 B5000                       US

 B6000                       US   

 A6000                       JP

When execute first line in CUST_MERGE, B5000 and B6000, return Y, as ctry_id are same (US) When execute second line in CUST_MERGE, B5000 and A6000, return N, as ctry_id are different (US,JP)

Use this query to get the flag:

SQL> WITH  CUST_MERGE( NEW_CUST_REF_ID, OLD_CUST_REF_ID) AS
  2    (SELECT 'B5000', 'B6000' FROM dual UNION ALL
  3     SELECT 'A5000', 'A6000' FROM dual),
  4  CUST_DETAILS (CUST_REF_ID,CTRY_ID) AS
  5   ( SELECT 'B5000', 'US' FROM dual UNION ALL
  6     SELECT 'B6000', 'US' FROM dual UNION ALL
  7     SELECT 'A5000', 'UK' FROM dual UNION ALL
  8     SELECT 'A6000', 'JP' FROM dual)
----------------------------
----End of Data Preparation
----------------------------
  9  SELECT cm.new_cust_ref_id,
 10         cm.old_cust_ref_id,
 11         dc.ctry_id,
 12         dc1.ctry_id,
 13         CASE
 14           WHEN dc.ctry_id = dc1.ctry_id THEN
 15            'Y'
 16           ELSE
 17            'N'
 18         END AS flag
 19    FROM cust_merge cm
 20    JOIN cust_details dc
 21      ON cm.new_cust_ref_id = dc.cust_ref_id
 22    JOIN cust_details dc1
 23      ON cm.old_cust_ref_id = dc1.cust_ref_id;

Output

NEW_CUST_REF_ID OLD_CUST_REF_ID CTRY_ID CTRY_ID FLAG
--------------- --------------- ------- ------- ----
B5000           B6000           US      US      Y
A5000           A6000           UK      JP      N

The query for your tables comes out to be

SELECT cm.new_cust_ref_id,
       cm.old_cust_ref_id,
       dc.ctry_id, 
       dc1.ctry_id,
       CASE
         WHEN dc.ctry_id = dc1.ctry_id THEN
          'Y'
         ELSE
          'N'
       END AS flag
  FROM cust_merge cm
  JOIN cust_details dc
    ON cm.new_cust_ref_id = dc.cust_ref_id
  JOIN cust_details dc1
    ON cm.old_cust_ref_id = dc1.cust_ref_id;

Update: With the new data, results are as expected:

SQL> WITH  CUST_MERGE( NEW_CUST_REF_ID, OLD_CUST_REF_ID) AS
  2    (SELECT 'B5000', 'B6000' FROM dual UNION ALL
  3     SELECT 'B5000', 'A6000' FROM dual),
  4  CUST_DETAILS (CUST_REF_ID,CTRY_ID) AS
  5   ( SELECT 'B5000', 'US' FROM dual UNION ALL
  6     SELECT 'B6000', 'US' FROM dual UNION ALL
  7     SELECT 'A6000', 'JP' FROM dual)
  8  SELECT cm.new_cust_ref_id,
  9         cm.old_cust_ref_id,
 10         dc.ctry_id,
 11         dc1.ctry_id,
 12         CASE
 13           WHEN dc.ctry_id = dc1.ctry_id THEN
 14            'Y'
 15           ELSE
 16            'N'
 17         END AS flag
 18    FROM cust_merge cm
 19    JOIN cust_details dc
 20      ON cm.new_cust_ref_id = dc.cust_ref_id
 21    JOIN cust_details dc1
 22      ON cm.old_cust_ref_id = dc1.cust_ref_id;

Output:

NEW_CUST_REF_ID OLD_CUST_REF_ID CTRY_ID CTRY_ID FLAG
--------------- --------------- ------- ------- ----
B5000           B6000           US      US      Y
B5000           A6000           US      JP      N