Kind of strange need with UNION

Question

Hello,
There are 2 tables located on 2 different dbs but the tables structure are the same.
I perform:
Select name, version, status, price, input date
from product@DB1
UNION
Select name, version, status, price, input date
from product@DB2
BUT if (name and version and status) are the same for 2 records coming from DB1 anb DB2, I want to have only the record coming from DB2.
Exemple:
If DB1 has a record with
"Engine 1.0 Available 10$ 21/01/2004"
and DB2 a record with
"Engine 1.0 Available 12$ 22/02/2005"
I want only the record "Engine 1.0 Available 12$ 22/02/2005" to be returned.
Having a group by is not enough as I can't choose the record displayed (?).
Any help is greatly appreciated.
Regards,
Guillaume.

Answer 1

Hello
if you use union all instead of union you will get rid of a sort and get a list of all records back from both queries. You can then use this as the result set to feed into another query which will let you select the rows you want. Something along these lines.
SELECT
    name,
    version,
    status,
    price,
    input_date
FROM
  SELECT
    name,
    version,
    input_date,
    price,
    db_id,
    row_number() OVER(PARTITION BY name,version,status ORDER BY db_id DESC) rn
  FROM
    (SELECT
          1 as db_id
          name,
          version,
          status,
          price,
          input_date
     from
          table@db1;
     UNION ALL
     SELECT
          2 as db_id
          name,
          version,
          status,
          price,
          input_date
     from
          table@db2;
) a
WHERE
    a.rn = 1;I've not been able to test it as of yet but the principal should work. If you only want prioritise the row based on which database it came from then use the db_id. It may be better though to use a not exists clause to prevent rows comming back from db1 where they already exist in db2. If you want to prioritise on something else, such as input date, you can change that:
row_number OVER(PARTITION BY name, version, status ORDER BY input_date)If you want to see where the row came from you can put the db_id in the outer query.
HTH
David