Detaching a database also alters file permissions

Moving a database or some of its files from one drive to another or from one instance of SQL Server to another is as simple as detaching it and re-attaching it again. This is actually pretty smart, compared to backuprestore, because you only perform one I/O operation (moving the file), as opposed to two (backing up, restoring).

But when you try to attach the database, you might get something like

Msg 5120, Level 16, State 101, Line 3
Unable to open the physical file "E:\Microsoft SQL Server\SQL2014\MSSQL\Data\Playlist.mdf".
Operating system error 5: "5(Access is denied.)".

The reason, as I found out the hard way, is that SQL Server can actually modify the file permissions of the .mdf and .ldf files when it detaches a database.

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Rebuilding a transaction log file

The database’s transaction log file contains, like the name implies, a log of all the transactions happening in the database. If you’ve set your database to the “simple” recovery model, the log is truncated (emptied) at every checkpoint (more or less all the time). In “bulk logged” or “full” recovery model, you’ll have to truncate the log files yourself, by performing a transaction log backup.

So because of this, the log file has to stay in sync with the rest of the database at all times. But what do you do if your log file is lost or corrupted? Luckily, there’s a simple way to recover it.

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