Good reminder. Not specific to MySQL, though. This happens in many languages that have unsigned integers. The underlying bit pattern resulting from twos-complement arithmetic is the same for signed and unsigned arithmetic, but when interpreted as unsigned you get one more high bit instead of that bit representing the sign: 0 for positive, 1 for negative.
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[ 2.8 ms ] story [ 19.2 ms ] thread"1 - 1" will correctly be computed as "0".