User Sa چیست و Mixed Mode چه حالتی میباشد؟

Authentication Modes
Microsoft® SQL Server™ can operate in one of two security (authentication) modes:

Windows Authentication Mode (Windows Authentication)
Windows Authentication mode allows a user to connect through a Microsoft Windows NT® 4.0 or Windows® 2000 user account.

Mixed Mode (Windows Authentication and SQL Server Authentication)
Mixed Mode allows users to connect to an instance of SQL Server using either Windows Authentication or SQL Server Authentication. Users who connect through a Windows NT 4.0 or Windows 2000 user account can make use of trusted connections in either Windows Authentication Mode or Mixed Mode.

SQL Server Authentication is provided for backward compatibility. For example, if you create a single Windows 2000 group and add all necessary users to that group you will need to grant the Windows 2000 group login rights to SQL Server and access to any necessary databases.

System administrator (sa) is a special login provided for backward compatibility. By default, it is assigned to the sysadmin fixed server role and cannot be changed. Although sa is a built-in administrator login, do not use it routinely. Instead, make system administrators members of the sysadmin fixed server role, and have them log on using their own logins. Use sa only when there is no other way to log in to an instance of Microsoft® SQL Server™ (for example, when other system administrators are unavailable or have forgotten their passwords).