Are there any well-known German expressions for someone who is very board and detailed in knowledge? for example:
He is a human database
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Sie ist ein wandelndes Lexikon.
Sometimes also used in this form
Sie ist ein lebendes Lexikon.
One option would be "der/die Universalgelehrte" from the English word "polymath".
The word I would use is "savant." It was originally French, may have been adopted in German, but I'm not sure.