You cannot attach SQL Server backup, you've to RESTORE it.

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>I asked a client to send me a backup of their SQL Server database. I received a .DAT file! Now, I'm not sure what I can do with this. I tried Attaching it to my SQL Server, but got the message that this is not a primary database file.
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>Is there anything I can do with this file? Or do I need to tell them to do a backup using the SQL Server Management Studio (or whatever is the equivalent for the version of SQL Server they have)?