DataLemur comes from Nick Singh and Ergest Xheblati, the pair behind the well known book Ace the Data Science Interview, and that pedigree shows in how the platform is put together. The headline is the free SQL question bank, which is large, well organised, and pulled from real company interviews, and you solve everything in the browser against actual data so you get immediate feedback on whether your query is right rather than just reading a solution. For anyone preparing for a data analyst or data scientist role where SQL is the gatekeeping skill, this is close to the ideal place to grind, and the fact that the core of it costs nothing is a genuine public service in a market full of paywalls. The questions are tagged by company and by difficulty, and the progression from gentle warm ups to properly hard window function and self join problems mirrors how real interview loops escalate.
The premium tier, which is a single payment rather than a subscription, unlocks fuller solution explanations and more of the data science and statistics material, and for many people the companion book pairs naturally with it. The limitations are mostly about scope rather than quality. This is a SQL first resource, and while there is data science and probability content, it is not a broad machine learning interview trainer, so candidates for heavy ML engineering roles will need to look elsewhere for model design and coding rounds. It is also a practice and reference tool rather than a taught curriculum, so a genuine beginner who does not yet know what a join is should spend a few hours with an introductory SQL tutorial before diving in, otherwise the questions will simply be frustrating.
My take is that DataLemur is one of the highest value resources in this whole category precisely because so much of it is free and so much of it is good. Start with the free bank, work through it seriously, and only reach for premium once you have proven to yourself that you will actually use it.