mysql - Read that becomes "dirty" - SQL Isolation Level -

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which of sql transaction isolation levels protect against read becomes dirty partway through transaction?

for instance, take schedule of 2 transactions, t1 , t2:

t1 reads a, t2 reads a, t1 writes a, t1 commits*, t2 writes a, t2 commits

after starred command t1 commits occurs, value read t2 kind of "dirty." read committed value @ time did read (assuming other long before transaction committed value a), value no longer reflective of in a. isolation level protect against this? not dirty read, since @ time of read, value of committed value. if t1 , t2 both updating value of a, lets say, adding 20 value, result @ end of schedule not correct... "serializable" protect against this, or other? wouldn't think repeatable read because no reads repeated, not sure...


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