Why should you care?

Swiggy cannot allow an order with a negative total or a line item pointing to a deleted restaurant — constraints catch that at insert time.

See it live — copy this example

Run in pgAdmin or psql.

CREATE TABLE orders (
  order_id bigserial PRIMARY KEY,
  customer_id bigint NOT NULL,
  total_amount numeric(10,2) CHECK (total_amount >= 0),
  status text NOT NULL DEFAULT 'pending'
);

What happened?

• bigserial auto-generates order_id.
• CHECK blocks negative totals.
• DEFAULT fills status when omitted.
• PRIMARY KEY implies UNIQUE and NOT NULL on order_id.

Practice next

1. Connect to PostgresVerse and CREATE TABLE orders as shown.
2. Insert a valid row with INSERT INTO orders (customer_id, total_amount) VALUES (1, 249.00);
3. Try INSERT with total_amount -1 and read the CHECK error.
4. Add UNIQUE (customer_id, created_at) after adding a created_at column.
5. Name a constraint explicitly: CONSTRAINT orders_total_nonneg CHECK (...).

Remember

Constraints are enforced on every INSERT/UPDATE. CHECK validates expressions; FK validates references. \d tablename lists constraints in psql.

PostgresVerse order guardrails

An intern API tries to POST total_amount=-50; PostgreSQL rejects it before the bug reaches finance.

Outcome: Product team adds CHECK constraints instead of duplicating validation in three services.