After authenticating a user, SQL Server will then determine whether the user has permission to view and/or update data, view metadata, or perform administrative tasks (server-side level, database-side level, or both). If the user, or a group to which the user is a member, has some type of permission within the instance and/or specific databases, SQL Server will let the user connect.

In a nutshell, authorization is the process of checking user access rights to specific securables. In this phase, SQL Server will check the login policy to determine whether there are any access rights to the server and/or database level. Login can have successful authentication, but no access to the securables. This means that authentication is just one step before login can proceed with any action on SQL Server.

SQL Server will check the authorization process on every T-SQL statement. In other words, if a user has SELECT permissions on some database, SQL Server will not check once and then...