这种问题一般是因为客户端不知道要连那个instance或者不知道要使用哪个TNS Alias
可能的原因1.服务没有起来下边是Tom的一个Test Case来说明OracleServiceSID服务没有起来的话，是会引起ORA-12560错误的。
C:\Documents and Settings\tkyte>sqlplus scott/tigerSQL*Plus: Release 10.1.0.4.0 - Production on Thu Sep 6 15:43:55 Copyright (c) 1982, , Oracle. All rights reserved.Connected to:Oracle Database 10g Enterprise Edition Release 10.1.0.4.0 - ProductionWith the Partitioning, OLAP and Data Mining optionsscott%ORA10GR1> exitDisconnected from Oracle Database 10g Enterprise Edition Release 10.1.0.4.0 - ProductionWith the Partitioning, OLAP and Data Mining optionsC:\Documents and Settings\tkyte>net stop oracleserviceora10gr1The OracleServiceORA10GR1 service is stopping.........The OracleServiceORA10GR1
