Sqlplus different effects of two sign-in modes

Windows 8,oralce11g, command-line

1. Input "Sqlplus", enter, prompt: Please enter user name, enter user name, return, prompt, enter password, enter password, return, Report ora-12560:tns: Protocol Adapter error

2. Enter "Sqlplus username/password @sid", enter, Prompt "Connect to: Oracle Database 11g Release 11.2.0.1.0-64bit Production".

What is the difference between the two mechanisms?

Someone online says:

"Encounter the same situation: with C:\>sqlplus" User/[email protected] "This way you can log in, but directly run Sqlplus and then enter the user name password will prompt the protocol adapter error. Tried online a lot of methods, now finally solved, is the environment variable path value there problem, I installed the server and the client, in the path value there to put the server address in front of the client is good. such as this: D:\oracle\product\11.2.0\dbhome_1\bin;d:\oracle\product\11.2.0\client_1\bin; C:\Program Files\arcgis\arcsde\ora11gexe\bin; "

Try to change position, it has no effect. I did not install the server, only installed the client, and the above situation is not quite consistent.

Pending study.

Sqlplus different effects of two sign-in modes