Consistency

Let’s understand how a database keeps data valid using an accounts table. Assume the table has a rule like: balance ≥ 0 This is a constraint which means the database wont allow any negative balance. Initial data Alice = 1000 Bob = 500 Trying to break the rule now suppose we try to deduct more money than Alice actually has UPDATE accounts SET balance = balance - 2000 WHERE name = 'Alice' ; Alice only has 1000 so this would make her balance -1000 which violates the constraint. Result: The database throws an error and rejects the update. Directly setting a negative value UPDATE accounts SET balance = - 500 WHERE name = 'Bob' ; Again this violates the rule. Result: The database blocks the query with an error. Checking the data again SELECT * FROM accounts; Alice = 1000 Bob = 500 invalid operations were rejected the data remains unchanged. Inside a transaction Now lets try the same invalid update within a transaction: BEGIN ; UPDATE accounts SET balance = balance - 2000 WHERE name = 'Alice'