我为什么需要其中两个？什么时候必须使用一个或者另一个？

解决方案

回答

请参阅：http://www.sap-img.com/oracle-database/finding-oracle-sid-of-a-database.htm

What is the difference between Oracle
  SIDs and Oracle SERVICE NAMES. One
  config tool looks for SERVICE NAME and
  then the next looks for SIDs! What's
  going on?!
  
  Oracle SID is the unique name that
  uniquely identifies your
  instance/database where as Service
  name is the TNS alias that you give
  when you remotely connect to your
  database and this Service name is
  recorded in Tnsnames.ora file on your
  clients and it can be the same as SID
  and you can also give it any other
  name you want.
  
  SERVICE_NAME is the new feature from
  oracle 8i onwards in which database
  can register itself with listener. If
  database is registered with listener
  in this way then you can use
  SERVICE_NAME parameter in tnsnames.ora
  otherwise - use SID in tnsnames.ora.
  
  Also if you have OPS (RAC) you will
  have different SERVICE_NAME for each
  instance.
  
  SERVICE_NAMES specifies one or more
  names for the database service to
  which this instance connects. You can
  specify multiple services names in
  order to distinguish among different
  uses of the same database. For
  example:
  
  SERVICE_NAMES = sales.acme.com,
  widgetsales.acme.com
  
  You can also use service names to
  identify a single service that is
  available from two different databases
  through the use of replication.
  
  In an Oracle Parallel Server
  environment, you must set this
  parameter for every instance.

简而言之：SID =数据库实例的唯一名称，ServiceName =连接时使用的别名

回答

@DAC

In short: SID = the unique name of
  your DB, ServiceName = the alias used
  when connecting

并非完全正确。SID = INSTANCE的唯一名称(例如，计算机上运行的oracle进程)。 Oracle将"数据库"视为文件。

服务名称=实例(或者许多实例)的别名。这样做的主要目的是，如果我们正在运行集群，则客户端可以说"将我连接到SALES.acme.com"，DBA可以即时更改可用于SALES.acme.com请求的实例数，甚至将SALES.acme.com移至完全不同的数据库，而客户端无需更改任何设置。

回答

根据Oracle词汇表：

SID is a unique name for an Oracle database instance. ---> To switch
  between Oracle databases, users must specify the desired SID <---. The
  SID is included in the CONNECT DATA parts of the connect descriptors
  in a TNSNAMES.ORA file, and in the definition of the network listener
  in the LISTENER.ORA file. Also known as System ID. Oracle Service Name
  may be anything descriptive like "MyOracleServiceORCL". In Windows,
  You can your Service Name running as a service under Windows Services.

我们应该在TNSNAMES.ORA中使用SID作为更好的方法。