Lesson 25: ALTER TABLE & Schema Migrations

Schemas evolve as requirements change — ALTER TABLE modifies an existing table's structure without losing existing data.

Real-time example

Adding a loyalty_points column to an existing customers table with a million rows, safely, in production.

ALTER TABLE customers ADD COLUMN loyalty_points INT DEFAULT 0;
ALTER TABLE orders ALTER COLUMN status SET NOT NULL;
ALTER TABLE orders RENAME COLUMN amt TO amount;

What's happening

Adding a column with a DEFAULT backfills existing rows with that value — on huge tables this can lock the table, so real migration tools do it carefully.

In production, schema changes are usually run through a migration tool (Flyway, Liquibase) so every environment applies them identically.