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A non-repeatable read occurs when one transaction reads a row, and then a second transaction modifies that row. If the second transaction commits its change, subsequent reads by the first transaction yield different results.
For example, Tran1 reads the row for product_no 10 and sets the product_balance to 120. Now Tran2 reads the same row, and updates it to 80 and commits. If Tran1 then reads the same row again, it gets 80, instead of 120. That means, reading the same row is resulting in two different values.
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