Thea
“I wanted clear tasks I could complete in sequence, then repeat with confidence. Joins were still the hardest part.”
From your first SELECT
to window functions.
Six courses built around real Postgres sandboxes. Read a short lesson, write the query, see your result graded in milliseconds.
6
courses
25
lessons
86
exercises
Start here
SQL Fundamentals
Write your first real queries — pick columns, filter rows, sort results, and limit output. The four moves you will use in every query you ever write.
5 lessons19 exercises~63 min
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Begin the course →
lesson 1 · selecting-columns
SELECT first_name, salaryFROM employeesORDER BY salary DESCLIMIT 5;
5 rows · 8 ms✓ passed
Continue the path
Hand-picked orderBeginner
02Filtering & Logic
Stop returning too many rows. Master WHERE clauses with AND, OR, LIKE, IN, BETWEEN, and NULL handling — the difference between a query that works and one that almost works.
Builds on: SQL Fundamentals
What you'll learn
- ✓Master AND / OR / NOT logic
- ✓Use IN, BETWEEN, LIKE, IS NULL
- ✓Build multi-condition filters
HR Database14 exercises to complete
4 lessons14 exercises
Start →Unlocks Silver CrestAvatar
Intermediate
03Aggregations & Grouping
Turn thousands of rows into meaningful numbers. COUNT, SUM, AVG, GROUP BY, and HAVING power every dashboard and business report you will ever build.
Builds on: Filtering & Logic
What you'll learn
- ✓COUNT, SUM, AVG, MIN, MAX
- ✓Group rows with GROUP BY
- ✓Filter groups using HAVING
Store Database14 exercises to complete
4 lessons14 exercises
Start →Unlocks Diamond CrestAvatar
Intermediate
04Joins
Query multiple tables at once. Master INNER, LEFT, RIGHT, and FULL OUTER joins — and finally understand which rows appear in your result and why.
Builds on: Aggregations
What you'll learn
- ✓INNER, LEFT, RIGHT & FULL joins
- ✓Join tables on matching keys
- ✓Multi-table queries
HR Database14 exercises to complete
4 lessons14 exercises
Start →Path toward Platinum CrestAvatar
Intermediate
05Subqueries & CTEs
Break hard questions into readable steps. Subqueries and WITH blocks let you compose multi-stage logic that would be impossible — or unreadable — in a single query.
Builds on: Joins
What you'll learn
- ✓Scalar & correlated subqueries
- ✓Build reusable CTEs with WITH
- ✓Nested query logic
Store & HR databases13 exercises to complete
4 lessons13 exercises
Start →Path toward Platinum CrestAvatar
Advanced
06Window Functions
The most powerful feature in SQL — and the most asked-about in data interviews. Rank rows, run totals, and compare each row to its neighbors without collapsing your result set.
Builds on: Subqueries & CTEs
What you'll learn
- ✓RANK, ROW_NUMBER, DENSE_RANK
- ✓Running totals with SUM OVER
- ✓LAG, LEAD & NTILE
HR Database12 exercises to complete
4 lessons12 exercises
Start →Unlocks Platinum CrestAvatar