The big picture flow (Spring + PostgreSQL)

1. The Blueprint (Your Code)
   • You have a User class in your project (e.g., a Java class with @Entity annotations). This class defines the fields: id, username, email, etc. This is the ultimate source of truth.
2. The Robot Carpenter (Framework on Startup):
   • You start your application.
   • Your framework (Spring/Hibernate) wakes up and reads your application.properties or application.yml file. It sees spring.jpa.hibernate.ddl-auto = update.
   • It then scans all your code for blueprints (classes marked with @Entity, like your User class).
   • It connects to your PostgreSQL database and compares the blueprints to the existing tables.
     • ddl-auto options in yaml
   • If it’s the 1st time running (no database)
     • The tables are missing, so it generates the CREATE TABLE users (...) and CREATE TABLE products (...) SQL commands from scratch and runs them
   • If it’s NOT the 1st time running (db alr exists)
     • Since it’s set to update, it generates and runs the necessary SQL commands (CREATE TABLE ..., ALTER TABLE ...) to make the database schema match your code.
3. The API Call (Postman):
   • You send a POST request to /users with Postman to create a new user.
   • Your application’s controller code receives this request.
   • Your service/business logic takes the JSON data from Postman, creates a new User object, and tells the framework, “Hey, please save this object to the database.”
   • The framework automatically translates that User object into an INSERT INTO users (...) VALUES (...) SQL statement and sends it to PostgreSQL.
4. The Database (PostgreSQL):
   • PostgreSQL receives the INSERT command and adds a new row to the users table. The data is now permanently stored.

So, what about your schema.sql file?

You have two main ways to manage your schema:

1. Automatic (The Robot): Use ddl-auto: update and let the framework manage everything based on your entity classes. This is great for fast development.
2. Manual (The Hand-drawn Blueprint): Create a schema.sql file with the exact CREATE TABLE statements you want. You run this file yourself to set up the database. In this case, you would set ddl-auto: validate or ddl-auto: none to prevent the robot from interfering with your manual work.