Oracle dblink

The Oracle docs note the syntax for creating an Oracle dblink as follows:

CREATE [ SHARED ] [ PUBLIC ] DATABASE LINK dblink
  [ CONNECT TO
    { CURRENT_USER
    | user IDENTIFIED BY password
      [ dblink_authentication ]
    }
  | dblink_authentication
  ]
  [ USING 'connect_string' ] ;

 Oracle has invested heavily in distributed database technology and the creation of a database link is very straightforward.  You specify the dblink name, the remote user to connect to, the password for the remote user and the TNS service name for the database link connection:

create public database link
  mylink
connect to
  remote_username
identified by
  mypassword
using 'tns_service_name';

You can also create a dblink to non-Oracle databases,  which describes the steps for a database link to MySQL:

Step 1:  Your first step is having installed the TRANSPARENT GATEWAY (it comes in as of the options when you install Oracle).

Step 2: You must have a user in the SQL Server.

Step 3: In the directory <ORACLE_HOME>\tg4msql\admin look for the file inittg4msql.ora and have the following options:

HS_FDS_CONNECT_INFO="SERVER=name_server;DATABASE=name_db"
HS_FDS_TRACE_LEVEL=OFF
HS_FDS_RECOVERY_ACCOUNT=user_sqlserver
HS_FDS_RECOVERY_PWD=pass_user_sqlserver

Step 4: Configure the listener and add the following
(SID_DESC =
(PROGRAM = tg4msql)
(SID_NAME = MSQL)
(ORACLE_HOME = C:\oracle1)
)
)

and in our tnsnames.ora add

MSQL=
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = servidor)(PORT = 1521))
)
(CONNECT_DATA =
(SID = MSQL)
)
(HS=OK)
)

where HS set up as a heterogonous service

Step 5: create a link to sql server.
create database link msql connect to user_sqlserver identified by password using 'MSQL';

Step 6: You can now use the database link to a foreign database:

select * from table@msql