Free SQL Practice (5-Week Plan, Real Postgres)
Free SQL prep usually means a tutorial site that ranks for the keyword and a paid product behind the third link. The catalog here is the product. 854 problems, every one open from the first visit, every difficulty tier on the same page, every company tag visible regardless of sign-in state. The page below is a 5-week prep plan against that catalog.
Free SQL prep usually means a tutorial site that ranks for the keyword and a paid product behind the third link. The catalog here is the product. 854 problems, every one open from the first visit, every difficulty tier on the same page, every company tag visible regardless of sign-in state. The page below is a 5-week prep plan against that catalog.
Know the patterns before the interviewer asks them.
5-week prep timeline
Volume scales week over week as the patterns get harder. Hour budget assumes 1-2 hours per session.
Week-by-week, with target outcomes
Each week lists the topics, the problem count, and a measurable success criterion. If you can't hit the criterion, repeat the week before moving on.
- ▸SELECT, WHERE, ORDER BY, basic GROUP BY
- ▸INNER JOIN and LEFT JOIN with single foreign key
- ▸First COALESCE and NULL handling problems
- ▸GROUP BY with HAVING, multiple aggregates
- ▸CASE WHEN inside SUM/COUNT (conditional aggregation pivot)
- ▸Cohort retention and funnel patterns
- ▸ROW_NUMBER for dedup and top-N per group
- ▸LAG/LEAD for period-over-period deltas
- ▸SUM OVER and AVG OVER with explicit ROWS frame
- ▸Sessionization (LAG + SUM-as-accumulator)
- ▸Gap-and-island for streaks and consecutive runs
- ▸Slowly changing dimension lookups with half-open intervals
- ▸Timed drill with 12-15 minute cap
- ▸AI mock interviewer once mid-week and once near the end
- ▸Targeted re-practice of patterns where completion rate is below 70%
The arithmetic argument for free
Why free changes practice volume, not just sticker price.
The argument for free isn't emotional. It's arithmetic. A candidate who needs 5 weeks of practice and is paying $35/month for LeetCode Premium and $79/year for DataLemur is at roughly $145 in subscription cost across the prep window. At that cost most candidates ration their attempts: they finish a problem instead of solving it 5 different ways to confirm fluency, they skip the spaced-repetition retry because they've "already done that one", they avoid the mock interviewer because each session feels like part of a quota.
Free changes the marginal cost of the next attempt to zero. Candidates submit more, re-attempt more, and rotate through more mocks. That's what compounds.
Submissions per user, real distribution
How users actually use the free catalog. The 'phone-screen ready' bucket is the largest; the 'career change' bucket exists because the catalog scales.
Free SQL prep FAQ
How do I know this is free for real?+
Will I run out of free attempts?+
What about the AI mock interviewer? Surely that costs something.+
How is free DataDriven SQL different from free LeetCode SQL?+
What's the prep plan if I have less than 5 weeks?+
Should I supplement with a paid SQL site?+
Pick week 1 and solve a problem
- 01
Active recall beats re-reading by 50%
Cognitive-science meta-reviews (Dunlosky et al., 2013) rank practice testing as a top-tier study technique, while re-reading and highlighting rank near the bottom
- 02
76% of hiring managers reject on the coding task, not the resume
From HackerRank's 2024 Developer Skills Report. Candidates who look strong on paper still fail the live screen if they haven't done timed, executable practice
- 03
Five problem shapes cover 80% of data engineer loops
Dedup, sessionization, top-N-per-group, slowly-changing dimensions, partition tricks. Writing the shapes by hand turns the unfamiliar into pattern recognition