Learning the importance of consistency in a DB

After understanding Atomicity, I explored another important concept: Consistency . Consistency ensures that the database always stays in a valid state(i.e no invalid data) Creating a base table CREATE TABLE accounts ( id SERIAL PRIMARY KEY , name TEXT NOT NULL , balance INT NOT NULL CHECK ( balance >= 0 ), last_updated TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); INSERT INTO accounts ( name , balance ) VALUES ( 'Christina' , 1000 ), ( 'Sharon' , 500 ); Trying to Break the Rule (Negative Balance) UPDATE accounts SET balance = balance - 1500 WHERE name = 'Christina' ; What Happened? PostgreSQL rejected it: ERROR: new row violates check constraint "balance >= 0" Balance does not change. Direct Invalid Update UPDATE accounts SET balance = - 100 WHERE name = 'Sharon' ; Again, it fails due to the CHECK constraint What I Learned The rule CHECK (balance >= 0) is enforced by the database schema PostgreSQL automatically blocks invalid data This is how Consistency is maintained