Question

In .NET, Is it possible to run a nested query from two separate Oracle databases?

E.g.

“SELECT my_value,
FROM table_in_database_1
WHERE my_value IN ( SELECT my_value
FROM table_in_database_2 )”

If so, how would I go about doing this?

Ultimately, this is an effort to overcome an “ORA-01795” error from using an “in” statement with over 1000 items in the conditional list, without having to break the query out into multiple “OR value IN” lists.

Answer 1

You might be able to get away with simply building a database link and joining over that link, but that can have performance issues. Something like this:

Build a link to database 2 on database 1.

CREATE DATABASE LINK db2 CONNECT TO user IDENTIFIED BY pw USING tns-alias;

Join your table on database 1 to the table on database 2:

SELECT my_value 
  FROM table_in_database_1 t1 JOIN table_in_database_2@db2 t2
       ON t1.my_value = t2.my_value

Depending on the performance of the link, you may opt to use a hybrid approach for this, involving both a database link and a temporary table.

Build a database link as above. Build a temporary table on database 1 that holds the values to be used in the subquery from database 2.

CREATE GLOBAL TEMPORARY TABLE db2_values (value VARCHAR2(20));

Copy the values from db2 to db1:

INSERT INTO db2_values
SELECT my_value 
  FROM table_in_database_2@db2;

Finally, join your database 1 table to the temporary table.

SELECT my_value
  FROM table_in_database_1 t1 JOIN db2_values t2 ON t1.my_value = t2.value;