Question 1

Is this free?

Accepted Answer

**The first 4 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

Why a dedicated date/time collection?

Accepted Answer

**Date logic is the third-most-failed SQL interview category** after joins and window functions, yet every major practice platform treats it as a footnote.

Real-world dashboards run on date arithmetic:

- cohorts by signup month
- aging buckets for overdue invoices
- month-over-month growth
- day-of-week traffic patterns

*This collection isolates date/time as the single skill across 5 progressive stages on production schemas.*

Question 3

Does this work for PostgreSQL too?

Accepted Answer

**Yes.** The collection is SQLite-first because SQLite is what we run in-browser.

Every pattern translates directly to PostgreSQL:

- `JULIANDAY` → `EXTRACT(epoch FROM date1 - date2)/86400` or `date1::date - date2::date`
- `strftime('%Y-%m', x)` → `TO_CHAR(x, 'YYYY-MM')` or `DATE_TRUNC('month', x)`

*Solution explainers call out dialect differences where they matter.*

Question 4

What about timezones?

Accepted Answer

**All datasets store timestamps in UTC.**

We deliberately *do not test timezone conversion* in this collection — that is a separate, complex topic best handled in a dedicated postgres/bigquery timezone collection.

**Every problem here assumes UTC.**

Question 5

Do I need to know window functions for Stage 5?

Accepted Answer

**Yes** — Stage 5 uses `LAG`, `SUM OVER`, and `AVG OVER ROWS BETWEEN`.

If you have not seen window functions before, work through **LDS SQL Window Functions 30** first.

*Stage 5 here applies window functions specifically to time-series — it does not teach window function syntax from scratch.*

Question 6

How long does it take to complete all 20?

Accepted Answer

**Most users complete in 5-7 days** solving 3-4 problems per day.

Total estimated time is **~9 hours** of focused practice.

Question 7

How does this compare to DataLemur, StrataScratch, or LeetCode for SQL date/time?

Accepted Answer

Date/time SQL drills exist everywhere — and they all break the moment you switch dialects in a real job.

**What LDS SQL Date & Time 20 gives you that they don't:**

- **Cross-dialect parity training** — the only collection that runs every problem in **both SQLite and PostgreSQL** side-by-side, with the gotchas explicitly called out (`JULIANDAY` vs `EXTRACT(EPOCH …)`, `strftime('%Y-%W')` vs ISO week numbering, NULL vs out-of-range vs invalid-date semantics)
- **A 5-stage curriculum** — extraction → arithmetic → bucketing → rolling windows → cross-dialect portability
- **Real timestamped data** on industry schemas — login sessions, order placements, message sends, playback sessions
- **Built-in SQL Visualizer + AI Mentor** for tricky date-bucket cases

*Catalogs drill date functions on one dialect. This collection prepares you to survive a job switch from Stripe (PG) to Airbnb (Snowflake) to Uber (Hive).*

LDS SQL Date & Time 20

About this collection

Your Learning Path

Why These 20?

Cross-Dialect Date Mastery

Real Timestamped Events

Time-Series Capstone

Skill Coverage

FAQ

Ready to Master SQL?

LDS SQL Date & Time 20

About this collection

Your Learning Path

Why These 20?

Cross-Dialect Date Mastery

Real Timestamped Events

Time-Series Capstone

Skill Coverage

FAQ

Ready to Master SQL?

LDS SQL Date & Time 20

About this collection

Your Learning Path

Why These 20?

Cross-Dialect Date Mastery

Real Timestamped Events

Time-Series Capstone

Skill Coverage

FAQ

Ready to Master SQL?

LDS SQL Date & Time 20 — 20 Curated SQL Problems

Problems included in LDS SQL Date & Time 20

LDS SQL Date & Time 20

About this collection

Your Learning Path

Why These 20?

Cross-Dialect Date Mastery

Real Timestamped Events

Time-Series Capstone

Skill Coverage

FAQ

Ready to Master SQL?