Creating a DB link if not created inside a stored procedure - oracle

Question

I'm trying to code a stored procedure in Oracle. I don't have a ton of experience with it and I'm running to an issue. The end goal of the procedure is to take data from one DB and put it into another in a different form. I have most of the procedure working it seems but I'm having issues with something that seems like it should be simple. At the beginning of the code, I would like to check to make sure a DB link is created. If not, then I want to create the db link.
This is what I put inside my procedure:
IF (select count(1) from ALL_DB_LINKS where db_link = 'DB_LINK.NAME.COM') = 0 THEN
CREATE DATABASE LINK LINK_NAME
CONNECT TO username IDENTIFIED BY password
USING 'SID';
END IF;
I know the link works because I've done it outside this and did a lookup with it. The error I get when I try to compile is this:
Encountered the symbol "CREATE" when expecting one of the following:
I've done all the googling I think I can do and I cannot figure out what I am doing wrong. To head off the other question I have, I've also tried to do it by putting in:
DECLARE test_count number;
select count(1) into test_count from ALL_DB_LINKS where db_link = 'DB_LINK.NAME.COM';
BEGIN
IF test_count = 0 THEN
CREATE DATABASE LINK LINK_NAME
CONNECT TO username IDENTIFIED BY password
USING 'SID';
END IF;
END;
But I get the same error. I'm also not sure if having a begin inside a begin will work. Any help would be a great... well, help.

Answer 1

Oracle compiles packages and in order to do that all objects referenced in code need to already exist. This includes objects referenced over a database link. Things will get marked invalid if the referenced objects don't exist. Usually a DBA will create and maintain such an environment using PL/SQL scripts and not stored procedures.
If you really want to anyway:
EXECUTE IMMEDIATE q'[ CREATE DATABASE LINK LINK_NAME
CONNECT TO username IDENTIFIED BY password
USING 'SID']';

Answer 2

If you really need to do this, you can create the link using dynamic SQL as Ed Gibbs suggested and Brian showed, as long as the link already exists at the point the procedure is created:
create database link test_link
connect to scott identified by oracle
using 'ORCL';
Database link created.
create or replace procedure p42 as
l_count number;
l_dummy dual.dummy%type;
begin
select count(*) into l_count
from all_db_links
where db_link = 'TEST_LINK';
if l_count = 0 then
execute immediate q'[
create database link test_link
connect to scott identified by oracle
using 'ORCL'
]';
end if;
select dummy into l_dummy from dual#test_link;
dbms_output.put_line('Value from remote database: ' || l_dummy);
end;
/
Procedure created.
exec p42;
Value from remote database: X
PL/SQL procedure successfully completed.
But the procedure won't compile if the link doesn't already exist:
drop database link test_link;
create or replace procedure p42 as
...
end;
/
Warning: Procedure created with compilation errors.
show errors
Errors for PROCEDURE P42:
LINE/COL ERROR
-------- -----------------------------------------------------------------
16/5 PL/SQL: SQL Statement ignored
16/5 PL/SQL: ORA-00942: table or view does not exist
Curiously it doesn't object if you create the procedure while the link exists, and then drop the link; the procedure stays valid, the dynamic SQL works to recreate the link, and the procedure runs successfully:
drop database link test_link;
Database link dropped.
select object_type, object_name, status from all_objects
where object_type = 'PROCEDURE' and object_name = 'P42';
OBJECT_TYPE OBJECT_NAME STATUS
--------------- --------------- ---------------------
PROCEDURE P42 VALID
exec p42;
Value from remote database: X
PL/SQL procedure successfully completed.
I would have expected dropping the link to invalidate the procedure, but apparently not, despite it appearing in all_dependencies. You can't recompile the procedure though:
drop database link test_link;
Database link dropped.
alter procedure p42 compile;
Warning: Procedure altered with compilation errors.
show errors
Errors for PROCEDURE P42:
LINE/COL ERROR
-------- -----------------------------------------------------------------
16/5 PL/SQL: SQL Statement ignored
16/5 PL/SQL: ORA-00942: table or view does not exist
In order to let you compile (or recompile) while the link does not exist, you'd need to reference the link using dynamic SQL as well:
drop database link test_link;
drop database link test_link
*
ERROR at line 1:
ORA-02024: database link not found
create or replace procedure p42 as
....
execute immediate 'select dummy from dual#test_link' into l_dummy;
dbms_output.put_line('Value from remote database: ' || l_dummy);
end;
/
Procedure created.
exec p42;
Value from remote database: X
PL/SQL procedure successfully completed.
drop database link test_link;
Database link dropped.
exec p42;
Value from remote database: X
PL/SQL procedure successfully completed.
drop database link test_link;
Database link dropped.
alter procedure p42 compile;
Procedure altered.
exec p42;
Value from remote database: X
PL/SQL procedure successfully completed.