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There is a sql like that
UPDATE xxx SET num = num -1 WHERE num > 0;
Will this be an atomic operation even if I have no transaction statement?
Can this sql ensure the num field always be non-negative?
If autocommit is enabled, yes it will be executed atomically. Every statement will be a single transaction unless autocommit is disabled.
It should be noted that autocommit is enabled by default, hence why START TRANSACTION is usually necessary to initiate a transaction.