Oracle database link. Check for existence or overwrite?

Question

I need to check if a database link already exists before I create one. How can I do that?

I am writing an SQL script that starts with this:

DROP DATABASE LINK mydblink

then I create one:

CREATE DATABASE LINK mydblink
CONNECT TO testuser
IDENTIFIED BY mypswd
USING 'mypersonaldb'

I will of course get an error in the first step if the database link doesn't exists. And if I omit the first step and just go ahead and create a db link, I will again get an error saying that it already exists with the same name.

What can I do in order to check if the the database link already exists?

Answer 1

SELECT COUNT(1)
  FROM dba_objects -- user_objects
 WHERE object_type = 'DATABASE LINK'
   AND object_name = 'ARGUS51P';

For example (untested):

declare
  l_link_cnt pls_integer := 0;
  l_sql varchar2(32767);
begin
  -- link creation sql (fill in details of how you want this created)
  l_sql := 'create public database link ...';

  select count(1)
  into l_link_cnt
  from dba_objects
  where object_type = 'DATABASE LINK'
  and object_name = 'SOME_LINK';

  -- create link if it doesn't exist yet
  if (l_link_cnt = 0) then
    -- create link 
    execute immediate l_sql;

  end if;

end;

Answer 2

Oracle has no way to test for existence before a DROP or CREATE. (Well, ok, you could write some PL/SQL, but, that's probably more trouble than it's worth.) In Oracle scripting, it's pretty standard to simply do both the DROP and the CREATE in a script. If the DROP errors out, so be it. It won't affect execution of the script.

-Mark