Question 1

Is this free?

Accepted Answer

**The first 5 problems (Stage 1) are completely free.**

Stages 2-5 require a **Premium subscription**, which gives you access to all **1,500+ problems** across 15 production-grade datasets.

Question 2

How is this different from LDS SQL 50?

Accepted Answer

**LDS SQL 50** is the broad foundation that touches every SQL pattern.

**LDS Joins 30** is the dedicated deep dive — *joins-only, 30 problems, 5 stages* of progressively harder join patterns.

If you struggled with the joins stage of SQL 50, or if joins are the specific thing tripping you up in interviews, **this is the focused practice you need.**

Question 3

How is this different from LeetCode SQL joins?

Accepted Answer

**LeetCode uses toy tables with 2-3 columns** and a single join opportunity per problem.

Our problems use **8-15 table production schemas** modeled after companies like *Stripe, Uber, and Netflix*.

You'll join **3-6 tables per query** and learn schema-reading fluency alongside join syntax.

Question 4

Why does Stage 4 cover INTERSECT and EXISTS instead of "just self-joins"?

Accepted Answer

Because real interviews test the **SEMANTIC** of *"asking the same table from two angles"* — **not the syntax** of literal self-joins.

Some problems are cleanest with a literal `FROM table a JOIN table b`. Others are cleaner with **INTERSECT**, **correlated EXISTS**, or **CTE pre-aggregation**.

We teach all four so you pick the right idiom in context, instead of forcing self-join syntax onto every problem.

Question 5

Do I need to know window functions or CTEs first?

Accepted Answer

**No.** Stages 1-3 use only `INNER JOIN`, `LEFT JOIN`, and standard `WHERE`/`GROUP BY`.

- **Stage 4** introduces CTEs as one of the same-table idioms
- **Stage 5** uses CTEs and aggregation

If you have not seen CTEs before, work through **LDS SQL 50** first or pick up the basics from any SQL reference — *the Stage 5 capstones explain the CTE structure inline.*

Question 6

What SQL dialect is used?

Accepted Answer

**SQLite by default** with **PostgreSQL mode** available.

The concepts translate directly to any dialect — *BigQuery, Snowflake, Redshift, MySQL.*

Question 7

How long does it take to complete all 30?

Accepted Answer

**Most users complete in 7-10 days** solving 3-4 problems per day.

Total estimated time is **~12 hours** of focused practice.

Question 8

How does this compare to DataLemur, StrataScratch, or LeetCode for SQL joins?

Accepted Answer

Catalogs have join sections — but they're drilling environments, not curricula.

**What LDS SQL Joins 30 gives you that they don't:**

- **A 5-stage join curriculum** — INNER → LEFT → multi-FK star → anti-joins → set operations & EXISTS
- **Industry-grade 8–15 table schemas** — the same multi-FK relational shape real production queries demand, not 2–3 column toy tables
- **Multi-join composition per problem** — most problems require 2–4 joins, not single-pattern drills
- **Both SQLite and PostgreSQL** on every problem — toggle dialects to see how the same join behaves
- **Built-in SQL Visualizer + AI Mentor** to unstick you when a join blows up unexpectedly

*If you want to drill `INNER JOIN`, any catalog will do. If you want to learn the join muscle memory that production queries actually exercise, this is the most realistic surface.*

LDS SQL Joins & Multi-Table 30

About this collection

Your Learning Path

Why These 30?

Joins, in Depth

Industry-Grade Multi-FK Schemas

Multi-Join Composition Per Problem

Skill Coverage

FAQ

Ready to Master SQL?

LDS SQL Joins & Multi-Table 30

About this collection

Your Learning Path

LDS SQL Joins & Multi-Table 30

About this collection

Your Learning Path

Why These 30?

Joins, in Depth

Industry-Grade Multi-FK Schemas

Multi-Join Composition Per Problem

Skill Coverage

FAQ

Ready to Master SQL?

LDS SQL Joins & Multi-Table 30 — 30 Curated SQL Problems

Problems included in LDS SQL Joins & Multi-Table 30

LDS SQL Joins & Multi-Table 30

About this collection

Your Learning Path