READ COMMITTED database isolation level in oracle?

Your solution should work: your first update will lock the whole table. If another transaction is not finished, the update will wait. Your second update will guarantee that only one row will have the value 1 because you are locking the table (it doesn't prevent INSERT statements however).

You could consider using a unique, function based index to let Oracle handle the constraint of only having a one row with activated flag set to 1.

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