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I am confused about how this works:

I was reading the SQL Server documentation on authentication here:

Choose an Authentication Mode

It says:

Windows Authentication is always available and cannot be disabled

This makes me think that ALL connections to SQL server NEED windows authentication (as it cannot be disabled), but it goes on to talk about the advantages of having only SQL Server authentication, for example:

Advantages of SQL Server Authentication

  • Allows SQL Server to support environments with mixed operating systems, where all users are not authenticated by a Windows domain.

So, can you connect to SQL server without Windows Authentication or not?

  • It depends on authentication mode you have selected during installation. SQL Server provides two: 1. Windows authentication mode 2. SQL Server and Windows authentication mode. If you select 1 you will only be able to make connections using windows account. If you select 2, it will allow both SQL and windows account. – SQLPRODDBA Apr 12 '16 at 5:18
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Windows Authentication is always available, SQL Server Authentication is only available if Mixed Mode is enabled.

During setup, you must select an authentication mode for the Database Engine. There are two possible modes: Windows Authentication mode and mixed mode.

So:

For an instance using Windows Authentication mode, connections can only be established using Windows Authentication.

For an instance using Mixed Mode, connections may be made using either Windows Authentication or SQL Server Authentication.

In other words, Windows Authentication being always available does not mean it is always used. A company may choose to use only SQL Server Authentication, even if Windows Authentication cannot be disabled (it will remain enabled but unused).

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