create public database link                            
dblink名connect to 目标数据库名identified by 目标数据库密码                 
using '                                  
 (DESCRIPTION =                                 
  (ADDRESS_LIST =                                
   (ADDRESS = (PROTOCOL = TCP)(HOST = ip)(PORT = 1521))       
  )                                       
  (CONNECT_DATA =                                
   (SERVICE_NAME = 实例名)                            
  )                                       
 )'  ;

 select * from a.client@dblink名;

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