A database is an organised collection of tables. SQL is the language you use to ask questions and give instructions.

Tables, rows, and columns:

customers table:
┌────┬──────────┬───────────────────────┬──────────┐
│ id │ name     │ email                 │ city     │
├────┼──────────┼───────────────────────┼──────────┤
│  1 │ Arjun    │ arjun@example.com     │ Mumbai   │
│  2 │ Priya    │ priya@example.com     │ Delhi    │
│  3 │ Rohan    │ rohan@example.com     │ Bangalore│
└────┴──────────┴───────────────────────┴──────────┘

• Table = a named grid of data (like one sheet in a workbook)
• Column = a field that every row has (name, email, city)
• Row = one individual record (one customer)

How tables connect — the relationship:

orders table:
┌────┬─────────────┬────────┬─────────┐
│ id │ customer_id │ total  │ status  │
├────┼─────────────┼────────┼─────────┤
│ 1  │      1      │ 2999   │ shipped │
│ 2  │      1      │ 499    │ pending │
│ 3  │      2      │ 1200   │ shipped │
└────┴─────────────┴────────┴─────────┘

orders.customer_id points back to customers.id. Arjun (id=1) has two orders. Priya (id=2) has one. No data is duplicated — Arjun's name lives in one place only.

💡 Key Insight: This is why it is called "relational" — the tables are related through shared keys, not because data is copied between them.

The spreadsheet analogy:

Most people start with spreadsheets — but databases are built for something spreadsheets cannot handle well:

What SQL actually does:

SQL (Structured Query Language) is a language for talking to a relational database. It is not a programming language like Python — it is a query language. You describe what you want, and the database figures out how to get it.

-- Plain English: "Show me the names of all customers in Mumbai"
SELECT name
FROM customers
WHERE city = 'Mumbai';

What is MySQL?

MySQL is a database management system — the software that stores your data and understands SQL. Other systems (PostgreSQL, SQLite, SQL Server) also use SQL, but with small dialect differences. This course uses MySQL.

💡 Key Insight: SQL has been around since 1974 and is still the most-used data skill in job listings. Learning it once gives you leverage across almost every industry.

SQL is a standard — MySQL is a product:

SQL (language standard)
  ├── MySQL       ← free, open-source, most popular for web apps
  ├── PostgreSQL  ← free, open-source, more powerful features
  ├── SQLite      ← lightweight, used in mobile apps / local files
  ├── SQL <TopicPreview slug="server">Server</TopicPreview>  ← Microsoft, common in enterprise Windows shops
  └── Oracle      ← enterprise, very powerful, expensive

All of these understand core SQL. The differences are in:

• Advanced features (window functions, JSON support, full-text search)
• Performance characteristics
• Licensing and cost
• Specific syntax for things like date functions

This course uses MySQL syntax:

-- MySQL date function:
SELECT NOW();           -- current timestamp
SELECT CURDATE();       -- current date only

-- These are MySQL-specific (PostgreSQL uses NOW() and CURRENT_DATE)

What stays the same across all databases:

-- This works in MySQL, PostgreSQL, SQLite, SQL Server, Oracle:
SELECT name, email
FROM customers
WHERE city = 'Mumbai'
ORDER BY name;

💡 Key Insight: Master MySQL first. Moving to PostgreSQL later takes about one day — 95% of what you learn here transfers directly.