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