Databases — CodeSensei Teaching Module

What is a Database?

• Analogy: A super-powered spreadsheet that your app can read and write to automatically. Each "table" is like a sheet, each "row" is an entry, each "column" is a field.
• Key insight: Without a database, your app forgets everything when it restarts. Databases make data PERMANENT.

SQL (Structured Query Language)

• Analogy: SQL is the language you use to talk to the database. Like asking a librarian: "Find me all books by this author, sorted by year."
• Essential operations:
  • SELECT — "Show me data" → SELECT * FROM users (show all users)
  • INSERT — "Add new data" → INSERT INTO users (name, email) VALUES ('Juan', 'juan@email.com')
  • UPDATE — "Change existing data" → UPDATE users SET name = 'Juan C' WHERE id = 1
  • DELETE — "Remove data" → DELETE FROM users WHERE id = 1
  • WHERE — "But only the ones that match this condition"
• Quiz: "Which SQL word would you use to find all users who signed up today?"

Types of Databases

• PostgreSQL — the reliable all-rounder. Most vibecoded apps use this. Think of it as Excel but 1000x more powerful.
• SQLite — a lightweight database stored in a single file. Good for small projects. Like a notepad vs a full filing system.
• MongoDB — stores data as flexible documents (JSON-like) instead of rigid tables. Like storing folders of papers vs a spreadsheet.

ORMs (Object-Relational Mappers)

• Analogy: An ORM is a translator between your code and the database. Instead of writing SQL directly, you write code in your programming language and the ORM converts it to SQL.
• Prisma (most common in vibecoded projects):
  • prisma.user.findMany() → becomes SELECT * FROM users
  • prisma.user.create({ data: { name: "Juan" } }) → becomes INSERT INTO users...
• Key insight: ORMs make databases easier to work with, but the SQL is still happening underneath.

Schemas & Migrations

• Schema — the blueprint of your database. Defines what tables exist and what columns each has.
• Migration — a change to the blueprint. Like remodeling a room — you write instructions for the change, then apply them.
• Analogy: The schema is your building's floor plan. A migration is the renovation permit that says "add a new room here."

Relationships

• One-to-Many: One user has many posts. Like one author writing many books.
• Many-to-Many: Users can have many tags, and tags can belong to many users. Like students and classes.
• Foreign Key: The link that connects tables. Like a reference number that says "this post belongs to user #5."