Earlier in this lesson we mentioned that SQL databases have a strict schema.

A schema is the structure of your database — it defines:

  • What tables exist
  • What columns each table has
  • What types each column uses (like numbers, text, dates)
  • What relationships exist between tables (like foreign keys - more on these in a future lesson)

For example, if you create this table:

CREATE TABLE users (
  id INT PRIMARY KEY AUTO_INCREMENT,
  username VARCHAR(100),
  email VARCHAR(255)
);

You’ve just defined part of your database schema.

In SQL, the schema is strict — if your table says a column must be a number, you can’t insert text.

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