SQL

Why does SQL exist?

Imagine you work at a store and your boss asks: "How much did each product category earn last month?"

Your data is in a spreadsheet. If you have a hundred rows, you can filter and scroll through it. But most real businesses have millions of rows — too many to scan by hand, too big for Excel to even open.

A database is a system built to hold that much data and search it fast. SQL is the language you use to ask it questions.

You write something like:

SELECT category, SUM(revenue)
FROM sales
WHERE month = 'January'
GROUP BY category

And in under a second, the database hands you exactly the answer. No scrolling. No pivot tables. No "my Excel crashed."

Almost all company data lives in a database. Sales records, user events, financial transactions, inventory — it is all there. SQL is the only language databases speak, which is why it is the first thing every data analyst learns.

What You'll Learn

• Writing queries to retrieve and filter data
• Aggregating and grouping records
• Joining multiple tables together
• Using subqueries and CTEs for readable, modular SQL
• Window functions for running totals, rankings, and comparisons

Lessons

Getting Started

Lesson	Concept
Introduction to SQL	What SQL is, how databases work, and your first queries

Your First Query

Lesson	Concept
SELECT	Retrieve columns from a table
FROM	Specify the source table and use aliases
WHERE	Filter rows by a condition
AND / OR	Combine multiple filter conditions

Filtering Techniques

Lesson	Concept
IN	Match against a list of values
BETWEEN	Filter by an inclusive range
LIKE	Pattern matching on strings
IS NULL / IS NOT NULL	Check for missing values

Shaping Results

Lesson	Concept
AS	Alias columns and tables for readability
DISTINCT	Remove duplicate rows
ORDER BY	Sort the result set
LIMIT / TOP	Restrict the number of rows returned

Joining Tables

Lesson	Concept
JOIN	Combine matching rows from two tables
ON	Define the join condition
LEFT JOIN	Keep all left-side rows, NULL-fill the right

Aggregating Data

Lesson	Concept
Aggregate Functions	COUNT, SUM, AVG, MIN, MAX
GROUP BY	Group rows for aggregation
HAVING	Filter groups after aggregation

How SQL Thinks

Lesson	Concept
SQL Execution Order	The order SQL actually processes your query

Writing Cleaner Queries

Lesson	Concept
CASE WHEN	Conditional logic in queries
WITH (CTEs)	Named temporary result sets

Window Functions

Lesson	Concept
Window Functions	OVER, PARTITION BY - compute across rows without collapsing them
ROW_NUMBER / RANK / DENSE_RANK	Ranking rows within a partition
LAG / LEAD	Access values from adjacent rows
Running Totals & Moving Averages	Cumulative sums, sliding windows, and frame specs

Set Operations & Subqueries

Lesson	Concept
Subqueries	Scalar, table, and correlated subqueries
UNION / INTERSECT / EXCEPT	Combine result sets vertically
EXISTS / NOT EXISTS	Test for row presence safely

Useful Functions

Lesson	Concept
COALESCE / NULLIF	Handle NULLs gracefully
Date & Time Functions	Truncate, extract, and compute with dates

Prerequisites

No prior experience required. A basic understanding of spreadsheets is helpful but not necessary.