Free SQL Practice Online
Free as in: no card on file, no email required to start, no Premium tier hiding the Hard problems. The whole catalog of 854 SQL problems is open at the same URL whether you sign in or not. The B2B side of the business pays for the Postgres instances; the practice site doesn't have to.
Free as in: no card on file, no email required to start, no Premium tier hiding the Hard problems. The whole catalog of 854 SQL problems is open at the same URL whether you sign in or not. The B2B side of the business pays for the Postgres instances; the practice site doesn't have to.
Know the patterns before the interviewer asks them.
What costs what, across the practice sites people actually use
Pricing reflects May 2026. The 'Easy' row is a useful tell: if a site rate-limits or paywalls Easy after the first one, the pattern continues higher up.
| Capability | DataDriven (this site) | LeetCode SQL | DataLemur | StrataScratch | HackerRank SQL |
|---|---|---|---|---|---|
| Easy SQL | All ~228 problems | Card on file required after 1 | Free | Free | Free |
| Medium SQL | All ~412 problems | Premium ($35/mo) | Premium ($79/yr) | Some free, most paid | Free (Postgres flavor varies) |
| Hard SQL | All ~214 problems | Premium | Premium | Premium | Mostly behind paywall |
| Company tags | 312 problems, 76 companies | Premium (most) | Premium | Premium | None |
| AI mock interviewer | Unlimited, no cap | Add-on | Premium add-on | Premium add-on | Not offered |
| EXPLAIN / query plan | Yes, every problem | Not offered | Not offered | Limited | Not offered |
| Spaced-repetition queue | Free | Premium | Premium | Premium | Not offered |
| Card required to sign in | Never | No (Premium upsell only) | No (Premium upsell only) | No (Premium upsell only) | No |
How to verify the 'no paywall' claim in 30 seconds
3 places to look. None of them require trusting the marketing copy on this page.
The pricing page is the proof. There's no SQL Premium plan. Anything on this site that runs SQL runs it at the same tier.
Open devtools, submit a query, watch the /grade call return 200. There's no entitlement check, no tier flag, no remote feature gate.
The privacy policy spells out the data flows. Queries submitted aren't sold or shared with recruiters. The B2B product is unrelated to the practice catalog.
Sign-up flow, complete
2 paths through the front door. Path A is anonymous and is what 78% of weekly active users take. Path B is opt-in sign-in for cross-device sync.
# Path A: anonymous (most users start here)
GET /problems?domain=sql
-> editor loads, schema panel renders, submit button live
POST /grade body={query: "SELECT ..."}
-> 200 OK, results return, progress stored in localStorage
# no email collected, no card, no account
# Path B: opt-in sign-in (for cross-device sync)
POST /auth/sign-in body={email: "you@example.com"}
-> magic link sent
GET /auth/callback?token=...
-> session established, localStorage progress merged into the account
# still no card, no payment method, no Premium promptSubmissions per day, last 30 days
The free catalog handles real volume. Every one of these submissions hit the same Postgres path that anonymous users get.
Weekend dips are visible. Every one of these submissions hit the same free Postgres path. No gating, no premium upsell at submission time.
The skeptical reader's objection list
4 reasonable questions about how a free site stays free, with concrete answers.
How is this sustainable?
The practice site is funded by a separate B2B product. Postgres replay containers cost $0.0003 per submission. With ~12,000 submissions a day at full traffic, the math is small enough that practice is a marketing channel, not a revenue stream that needs to monetize directly.
What if you start charging later?
If a paid tier ever exists, problems that were free will stay free for users who solved them before the change. Forward grandfathering, in the engineering blog when it ships. The current 854 problems will not be moved behind a paywall.
What's the catch with the AI mock interviewer?
There isn't one. The model calls cost roughly $0.04 per mock session at current pricing. Unlimited mock sessions per user. The hard rate limit is 1 concurrent session per IP, which exists to keep the inference queue responsive.
Do you sell the queries I write?
No. Submitted SQL is retained for grading and for catalog analytics (which problems get the most failed submissions, which seeds are too easy or too hard). The queries are not exposed to other users, recruiters, or third-party trainers. The privacy policy is the binding version.
Questions about the 'free' part
Is this really free or is there a catch I'm missing?+
How does this compare to LeetCode SQL's free tier?+
Will the free version get downgraded later?+
Do I have to make an account?+
What data do you collect?+
What's actually paywalled anywhere on the site?+
Open a problem and start typing
- 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