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When executing multiple queries simultaneously that modify the same data in different transactions, the following risks arise:

• Race Condition: changes made by one transaction can overwrite changes made by another, leading to data loss.

• Data integrity violation: if transactions are not properly isolated, data can remain in an inconsistent state.

• Phantom reads: one transaction sees rows modified or added by another transaction.

• Deadlocks: transactions can block each other, waiting for resources to be released.

• Lost updates: when two queries read the same data and then attempt to update it, one of the changes can be overwritten by the other.

To prevent these problems, transaction isolation levels and locking mechanisms are used, such as SELECT FOR UPDATE or optimistic locking.