14 parts

SQL & Databases

Learn SQL from zero: SELECT, filtering, joins, aggregates, subqueries, and schema design — every query runs live against a real database in your browser.

Why Learn SQL in 2026 (and What a Database Is)

What SQL and relational databases are, why every app needs one, and your first live query, runnable in your browser against a real database.

7 min read

SQL SELECT Basics: Querying Your Data

The SQL SELECT statement, explained: choose columns, use SELECT *, rename with AS, and remove duplicates with DISTINCT, runnable live in your browser.

9 min read

SQL WHERE: Filtering Rows with Conditions

Filter rows in SQL with WHERE: comparison and logical operators, BETWEEN, IN, LIKE, and IS NULL, with live, runnable queries in your browser.

8 min read

SQL ORDER BY, LIMIT and DISTINCT

Sort and trim SQL results: ORDER BY ascending/descending, sorting by multiple columns, LIMIT and OFFSET for paging, with live runnable queries.

8 min read

SQL Aggregate Functions: COUNT, SUM, AVG

Summarize data in SQL with aggregate functions (COUNT, SUM, AVG, MIN, MAX) and understand how they collapse many rows into one, with live queries to run.

7 min read

SQL GROUP BY and HAVING, Explained

Group rows and summarize per group with SQL GROUP BY, then filter groups with HAVING, and learn why HAVING isn't WHERE. With live, runnable queries.

8 min read

SQL Joins: INNER, LEFT and How They Work

Combine tables in SQL with joins: INNER JOIN, LEFT JOIN, join keys, and joining three tables, explained with a diagram and live, runnable queries.

8 min read

SQL Subqueries and Nested Queries

Use a query inside a query: SQL subqueries in WHERE with IN and EXISTS, scalar subqueries, and subqueries in FROM, with live, runnable examples.

9 min read

SQL INSERT, UPDATE and DELETE

Change data in SQL: INSERT new rows, UPDATE existing ones, and DELETE safely, plus why a missing WHERE is dangerous. With live, runnable examples.

8 min read

Creating Tables and Schema in SQL

Design tables with SQL: CREATE TABLE, column data types, PRIMARY KEY, NOT NULL, DEFAULT, and UNIQUE constraints. Build and query a table live in your browser.

10 min read

Database Relationships and Foreign Keys

Model relationships in SQL: one-to-many and many-to-many, foreign keys, and how the orders/customers/products tables connect, with a diagram and live queries.

9 min read

SQL Indexes and Query Performance

Make SQL queries fast: what an index is, when to add one with CREATE INDEX, reading EXPLAIN QUERY PLAN, and the trade-offs, explained simply, with examples.

8 min read

SQL Window Functions, Explained Simply

Window functions in SQL without the headache: OVER, PARTITION BY, ROW_NUMBER, RANK, and running totals. Keep every row while you rank and total, live.

9 min read

SQL Project: Design and Query a Database

Put it together: design a small library database with SQL, create the tables, add data, and answer real questions with joins and aggregates. Fully runnable.

11 min read