What is SQL?

SQL (Structured Query Language) is the standard language for working with data stored in relational databases. You use it to ask a database questions, such as "how many orders did we get last month?", and get the answer back as a table. Almost every company that stores data, from Flipkart-scale e-commerce to a small clinic, runs on a database that understands SQL.

SQL is usually pronounced "S-Q-L" or "sequel". Both are fine.

What is a database?

A relational database stores data in tables. A table looks like a spreadsheet:

• Each row is one record (one customer, one order).
• Each column is one attribute of that record (name, city, order date).
• Tables are linked to each other by shared columns called keys. For example, every order has a customer_id that points to a row in the customers table.

Popular relational databases include PostgreSQL, MySQL, Microsoft SQL Server, Oracle, Snowflake and Google BigQuery. They all speak SQL, with small differences in syntax called dialects.

Your first SQL query

Throughout these tutorials we use ShopKart, a small Indian e-commerce store. Here is a query that lists three customers:

SELECT customer_name, city
FROM customers
LIMIT 3;

Result:

(3 rows)

Read it almost like English: select the name and city from the customers table, and limit the output to 3 rows.

Here is a slightly more useful one: how many orders has ShopKart received?

SELECT COUNT(*) AS total_orders
FROM orders;

Result:

(1 row)

You will learn exactly how COUNT(*) and AS work in the coming lessons.

What can you do with SQL?

SQL commands fall into a few families:

As a data analyst you will spend most of your time on SELECT: filtering, grouping, joining tables and calculating metrics.

How SQL works: declarative, not step by step

In Python you write how to do something: loop over rows, check a condition, add to a total. In SQL you describe what you want, and the database engine works out the fastest way to get it.

-- "Give me total revenue from delivered orders"
SELECT SUM(oi.quantity * oi.unit_price)
FROM order_items oi
JOIN orders o ON o.order_id = oi.order_id
WHERE o.status = 'Delivered';

You never tell the database which rows to read first or how to match the tables. That is the job of its query optimizer.

SQL syntax basics

• SQL keywords are not case-sensitive: SELECT, select and Select all work. By convention we write keywords in capitals.
• Statements end with a semicolon ;.
• Text values go in single quotes: 'Bengaluru'. Double quotes are for column or table names.
• Comments start with -- (one line) or are wrapped in /* ... */ (multi-line).
• Line breaks and extra spaces don't matter, so format queries to be readable.

Common mistakes

• Using double quotes for text. WHERE city = "Delhi" fails in PostgreSQL because double quotes mean a column name. Use 'Delhi'.
• Expecting a fixed row order. Without ORDER BY, the database can return rows in any order.
• Learning a dialect instead of SQL. Focus on standard SQL first; dialect differences are small and easy to look up.

Practice question

Write a query that shows the product_name and price of 5 products.

SELECT product_name, price
FROM products
LIMIT 5;

Key takeaways

• SQL is the standard language for querying relational databases.
• Data lives in tables made of rows and columns, linked by keys.
• SELECT ... FROM ... is the starting point of almost every query.
• SQL is declarative: describe the result, and the database figures out how.