Question 1

Do I need LDS SQL 50 first?

Accepted Answer

**Helpful but not required.**

If you can write basic `SELECT`/`JOIN`/`GROUP BY` and have seen the `WITH` keyword in passing, *start here*.

Stage 1 begins with simple scalar subqueries to ease you in.

Question 2

Why start with subqueries before CTEs?

Accepted Answer

**CTEs are a more readable syntactic alternative to subqueries.**

To understand *WHY* CTEs help, you first need to feel the pain of nested subqueries.

Stage 1 (5 problems) gives you that taste; Stage 3 onwards shows how CTEs make the same queries cleaner.

Question 3

How is this different from LeetCode's CTE problems?

Accepted Answer

**LeetCode's CTE problems use 3-4 column toy tables.**

Our 25 problems run on **8-15 table production schemas** modeled after real companies.

You'll write CTEs in the context of *realistic multi-table queries* — **exactly the shape interviews use.**

Question 4

What about recursive CTEs?

Accepted Answer

**Honest answer:** our reference solution pool currently has *zero recursive CTE examples*.

Recursive CTEs are **uncommon in real interviews** (most pipelines don't need them), so we don't claim what we don't teach.

*The 25 problems here cover 95%+ of real interview CTE content.*

Question 5

How long to complete?

Accepted Answer

**~10 hours of focused work.** Most users finish in 1-2 weeks at 2 problems per day.

**Stage 4 (Multi-CTE Pipelines) is the densest** — give yourself ~25 minutes per problem there.

Question 6

What SQL dialect?

Accepted Answer

**SQLite-compatible by default**, with **PostgreSQL mode** available.

All CTE syntax works *identically on BigQuery, Snowflake, Redshift, and SQL Server.*

Question 7

How does this compare to DataLemur, StrataScratch, or LeetCode for CTEs and subqueries?

Accepted Answer

CTEs are taught everywhere — usually as the first tool. We do the opposite.

**What LDS SQL CTE & Subqueries 25 gives you that they don't:**

- **A subqueries-first pedagogy** — start with `EXISTS`, `NOT IN`, scalar and correlated subqueries (Stages 1–2), THEN move to single CTE → multi-CTE → multi-step scorecards (Stages 3–5)
- *You'll see why CTEs win on readability* — every CTE problem can be expressed as a subquery, and the contrast is the lesson
- **Multi-CTE business scorecards** on 8–15 table industry schemas — the multi-step pipelines real DS/DA queries demand
- **Both SQLite and PostgreSQL** on every problem
- **Built-in SQL Visualizer** that steps through each CTE individually

*Catalogs teach you to write CTEs. This collection teaches you when CTEs are actually the right tool.*

LDS SQL CTE & Subqueries 25

About this collection

Your Learning Path

Why These 25?

Subqueries First, CTEs Second

Multi-CTE Business Scorecards

Both Dialects, Same Editor

Skill Coverage

FAQ

Ready to Master SQL?

LDS SQL CTE & Subqueries 25

About this collection

Your Learning Path