Note that the ANSI/ISO SQL standard defines transaction-level characteristics, not just individual statement-by-statement-level characteristics. I'll examine transaction-level isolation, not just statement-level isolation.
The SQL isolation levels are defined based on whether they allow each of the preceding phenomena. It's interesting to note that the SQL standard doesn't impose a specific locking scheme or mandate particular behaviors, but rather describes these isolation levels in terms of these phenomena—allowing for many different locking/concurrency mechanisms to exist
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