Learn SQL

Quick reference and in-depth lessons to sharpen your SQL skills.

beginner5

intermediate9

advanced7

·21 lessons total

You're learning PostgreSQL

All queries run on real PostgreSQL — the world's most advanced open-source relational database. Syntax like FILTER (WHERE …), ::cast, and NULLIF() are all fair game.

Beginner

🔍

SELECT, WHERE & ORDER BY

The foundation of every SQL query — pick columns, filter rows, sort, and limit results.

LessonCoursePractice

🔢

Data Types & Casting

Understand PostgreSQL types (text, integer, numeric, date) and convert between them with :: or CAST().

✂️

String Functions

Clean and reshape text data with UPPER, LOWER, TRIM, CONCAT, SPLIT_PART, and case-insensitive matching.

❓

NULL Handling

Why NULL breaks comparisons and how to fix it with COALESCE, NULLIF, and IS NULL.

📊

Aggregation & GROUP BY

Collapse rows into summaries with COUNT, SUM, AVG, MIN, MAX and group them by category.

🔗

JOINs Deep Dive

Combine rows from multiple tables with INNER, LEFT, RIGHT, and FULL OUTER JOIN.

✏️

INSERT, UPDATE & DELETE

Write rows with INSERT, change them with UPDATE, remove them with DELETE — the DML triad every application depends on.

Intermediate

🔀

CASE Expressions

Add if-else logic to queries — bucket values, conditionally aggregate, and create custom sort rules.

🪆

Subqueries & EXISTS

Nest queries inside queries, use EXISTS for existence checks, and write correlated subqueries.

📋

Common Table Expressions

Simplify complex queries with WITH clauses, chain multiple CTEs, and use WITH RECURSIVE for hierarchies.

⭕

Set Operations

Combine or compare result sets with UNION, INTERSECT, and EXCEPT — the SQL Venn diagram.

📅

Date & Time Functions

Use EXTRACT, DATE_TRUNC, AGE, and interval arithmetic for time-series grouping and tenure calculations.

Advanced

🪟

Window Functions

OVER(), PARTITION BY, ROW_NUMBER, RANK, LAG/LEAD — calculations across related rows without collapsing them.

🔗

Advanced JOINs

Self-joins for hierarchies, anti-joins for missing rows, CROSS JOIN for combinations, and LATERAL for per-row subqueries.

⚡

EXPLAIN & Query Performance

Read query plans, spot sequential scans, add indexes, and turn slow queries into fast ones.

🌳

Recursive CTEs

Use WITH RECURSIVE to walk org charts, prerequisite chains, and any tree or graph stored in a single table.

📐

GROUPING SETS, ROLLUP & CUBE

Generate subtotals, grand totals, and multi-dimensional summaries in a single query.

Every lesson, one page each

In curriculum order, from your first SELECT to higher-ed analytics.