SQL exclude records found in

Question

I have the following SQL, how do I change the second JOIN, to exclude the same record already found in the first select statement. I don't want to add the same record twice (which could happen, because I might return different Marketing_channel information per join) ie how do I add the "not exist" code to the second join. I am having syntax issues doing this. Thanks in Advance
sel A.PARTY_ID, A.cust_prod_sub_id, A.EVENT_DT, B.Marketing_channel from
(Select
PARTY_ID,
cust_prod_sub_id,
event_dt,
SALES_DEALER_NO,
SALES_AGENT_NO
From
tabletwo ) as A
JOIN
(Select MARKETING_CHANNEL, SALES_AGENT_NO
From tableone
Where dealer_flag = 1
) as B
on a.SALES_DEALER_NO = b.SALES_AGENT_NO
UNION
sel A.PARTY_ID, A.cust_prod_sub_id, A.EVENT_DT, B.Marketing_channel from
(Select
PARTY_ID,
cust_prod_sub_id,
event_dt,
SALES_DEALER_NO,
SALES_AGENT_NO
From
tabletwo ) as A
JOIN
(Select MARKETING_CHANNEL, SALES_AGENT_NO
From tableone
Where agent_flag = 1
) as B
on a.SALES_agent_NO = B.SALES_AGENT_NO
--(I am guessing I have to add not exist statement here)

Answer 1

You want something like this?
WITH T1 AS  (SELECT 1 ID, 1 DAT FROM DUAL
   UNION ALL SELECT 2, 2 FROM DUAL
   UNION ALL SELECT 3, 3 FROM DUAL),
         T2 AS  (SELECT 2 ID, 1 DAT FROM DUAL
   UNION ALL SELECT 3, 2 FROM DUAL
   UNION ALL SELECT 4, 3 FROM DUAL)
SELECT 1 SRC, T1.*
FROM T1
WHERE T1.ID = :ID
UNION
SELECT 2 SRC, T2.*
FROM T2
WHERE T2.ID = :ID
AND NOT EXISTS
  (SELECT 1
   FROM T1
   WHERE T1.ID = :ID)for all values of ID between 1 and 3 it returns just the record from table T1. when ID = 4 it returns the record from T2.