ACID (Consistency)

The state of before, during or after a transaction needs to be consistent and valid

For example, considering the following transaction of money transfering from Alice to Bob

ALICE.BALANCE = 1000
BOB.BALANCE = 800

BEGIN TRANSACTION
	ALICE.BALANCE - 500
	BOB.BALANCE + 500
END TRANSACTION

So in a consistent environment, if we have a query of QUERY_SUM(ALICE, BOB) that query the sum of balance of both Alice and Bob happening in the background it has to be 1800 at all the time even during the transaction.

So

As a result, we can see that in Thread2, doesn't matter when we perform QUERY_SUM, result is still consistent and valid