当用户要跨本地数据库，访问另外一个数据库表中的数据时，本地数据库中必须创建了远程数据库的dblink,通过dblink本地数据库可以像访问本地数据库一样访问远程数据库表中的数据。

1 check the privilege of user

select * from user_sys_privs t

　　where t.privilege like upper('%link%');

there are three kinds about it:

1) SYS CREATE DATABASE LINK NO

2) SYS DROP PUBLIC DATABASE LINK NO

3) SYS CREATE PUBLIC DATABASE LINK NO

2 Create the DBLINK

After you have created a database link, you can use it to refer to tables and views on the other database. In SQL statements, you can refer to a table or view on the other database by appending @dblink to the table or view name. You can query a table or view on the other database with the SELECT statement. You can also access remote tables and views using any INSERT, UPDATE, DELETE, or LOCK TABLE statement.

Defining a Public Database Link: Example The following statement defines a shared public database link named remote that refers to the database specified by the service name remote:

CREATE PUBLIC DATABASE LINK remote 
   USING 'remote'; 

shell script, e.g:

#Read the admin database name, log into catalog, create the link to admin

cat $admin_list |

while read ADMIN

do

echo "Creating database link from " $CAT " to " $ADMIN "...
"

sqlplus -s $USER/$PASS@$CAT << DONE

create database link $ADMIN using '$ADMIN';

exit;

DONE

echo "Creating database link from " $ADMIN " to " $CAT "...
"

sqlplus -s $USER/$PASS@$ADMIN << DONE

create database link $CAT using '$CAT';

exit;

DONE

done

3 Use it

dsdsun02% sqlplus bsnl2010/arbor123@safx00m1

SQL*Plus: Release 10.2.0.1.0 - Production on Mon Jun 17 03:17:19 2013

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Connected to:

Oracle9i Enterprise Edition Release 9.2.0.7.0 - 64bit Production

With the Partitioning option

JServer Release 9.2.0.7.0 - Production

SQL> select max(account_no) from cmf@safx00m3

  2  ;

MAX(ACCOUNT_NO)

---------------

      598308961

参考：http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_5005.htm