Create Database DB Link in Oracle

What is DB Link?

A Database Link (DB Link) is a connecting string between two different physical database servers which allow a client to access them as logical database.

In simple word , If you are logged into one database and want to access only few tables , objects or schema of different database then DB link string will provide you access to that particular objects, table etc.

Following data dictionary views can give you basic information about the db link.

query-

 SQL> desc dba_db_links
 Name                                      Null?                         Type
 ----------------------------------------- -------- ----------------------------
 OWNER                                     NOT NULL       VARCHAR2(30)
 DB_LINK                                   NOT NULL       VARCHAR2(128)
 USERNAME                                                         VARCHAR2(30)
 HOST                                                                     VARCHAR2(2000)
 CREATED                                   NOT NULL       DATE


Pre-requisite information for DB Link:

1. Owner
2. DB_LINK name
3. Username and Password
4. HOST TNS details

How to create db link strings:

I need to create a db link in database name "India" to database name "Development" then we should have all pre-requisite information of database "Development"

CREATE DATABASE LINK  IND2DEV CONNECT TO
                 <User_Name> IDENTIFIED BY <PWD>
      USING
 '(DESCRIPTION =(ADDRESS = (PROTOCOL = TCP)(HOST=development.com)(PORT = 1524)) (CONNECT_DATA =(SERVER = DEDICATED) (SERVICE_NAME = DEV)))';

Database link created.

How to check DB link created successfully:

select sysdate from dual@IND2DEV;

SYSDATE
------------------
04-MAY-18

Note: If you get sysdate output then db link created successfully.