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I restored a backup file (it's used as a template for new databases) as a new database. Normally as part of this process, I take a full backup after the restore, but this time I was called away on another issue before I could do that and in the meantime, my regularly scheduled differential backup job ran. It successfully took a differential backup of this database without throwing any errors. Conversely, the log backup job failed correctly reporting there was no full backup available.

I have read in multiple places that you cannot take either a differential or a log backup without a full, so why is this particular database allowing a diff? Has anyone else ever encountered this behavior?

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It may seem counter-intuitive, but it's counting the full you restored it from as the last full backup.

This is a really good diagram to show what I'm talking about.

enter image description here

I got that image from this article on restore chains and not breaking them, and the rest of the article is also really good. (In short: Copy only is your friend.)

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    You'll even see evidence of the restored database's backup status in the backup* take in msdb; the information is imported from the backup header on restore.
    – Ben Thul
    Mar 14, 2015 at 15:36

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