Answers to some of the most common questions.


Common general questions.

4 answers

Hi there, 

My name is Leon Wei, most recently I am a senior data science and machine learning manager at Apple, in Silicon Valley.

I’ve been using SQL for over 10 years, and it is still my go-to programming tool to prepare data.

Before my current role, I worked as a research scientist at Amazon, where I focused on building large-scale real-time pricing optimization engine. 

I recently launched the Cracking the SQL Interview for Data Scientists course, and I need a platform for my students to easily follow the course and be able to practice SQL interview coding problems, and I can't find a solution online, so I created SQLPad.

The nice thing about sqlpad.io is that everything can be done in a browser, and my students don't have to waste hours to configure the database on their own computers.

I created both Fullstack Data Scientist School (datascientistschool.com) and sqlpad.io.

Unfortunately NO.

I have designed and created the entire SQL course and curated 80 SQL coding questions single-handedly, which took many long hours of hard work during nights and weekends, therefore you are not allowed to use any of those questions outside of this website, without my permission.

If you represent a company and are interested to acquire a license to use my interview problems for your hiring process, please feel free to contact me.

The Cracking SQL Interview course is a comprehensive course but is really focused on improving your SQL interview skills, with tips and techniques that I've learnt from interviewing thousands of data professional candidates. It saves time by concentrating on things that really matter.

We have seen satisfied customers ranging from fresh college STEM students searching their first industry jobs, and young professionals who want to switch their career track to work on data science.

SQLPad and the SQL interview course significantly improved their hands-on data processing capabilities, with laser-focused learning materials, to help land their dream job offers.


Common technical questions.

4 answers

Good question, I created the solutions in a specific style/format (e.g., using IN instead of JOIN), so my students who take the Cracking the SQL interview course can follow along.

For example: for the first 30 questions, none of the solutions are using JOINS, simply because students have not learned JOINs yet.

Of course, if you have a better solution, please feel free to let me know and I might replace my solution with yours!


SQL is a must-know programming language for any analytics track data scientists, it is the lingua franca for processing and managing data in the industry. I’ve been using SQL for many years, and it is still my go-to language to prepare and manage data.

However, it is not a strong-typed language, and there are many popular database systems with different syntaxes and built-in functions, it could be very confusing for first-timers.

As a hiring manager or part of the hiring committee, I often ask a lot of SQL questions during a data scientist job interview, and to make sure the candidate will be hands-on at work.

However, in my 15 years career, I have met so many fresh college graduates or young professionals starting their job searches without solid coding skills in SQL, and in the end, they didn’t get a job offer.

The SQL interview can bear other names and may be called  Technical Analysis or Data Interview during a FAANG company interview, you might be asked to perform a series of SQL operations to extract data and insights, and answer follow-up questions about their products.

(*) FAANG: Facebook, amazon, apple, Netflix and google

Great question, SQLPad's online playground, and the coding challenge's online judge is currently based on the latest Postgres 13.

Its query syntax is similar to other databases such as MySQL or Microsoft SQL Server, one of the major difference is on Postgres's datetime related functions.

More details can be found here: https://www.postgresql.org/docs/current/functions-datetime.html

For example, there is no datediff function in Postgres, but you can subtract one datetime column from another directly using the minus '-' operator, which is actually quite convenient and easier to type once you get the hang of it.

If you would like to see other database engines implemented on SQLPad, please let us know.



There is definitely a lot to prepare for your first job hunt, especially given the current COVID-19 situation, hiring managers to receive a lot more applications from qualified candidates, and it pushes the hiring bar significantly higher than before.

If you are interested to know more about how to prepare a data scientist interview, I wrote a blog to explain different types of data scientists tracks.

And if you are interested in finding a job search mentor to help you get started with a concrete plan. Feel free to hire me.


Common account questions.

2 answers

Absolutely, congratulations and we totally understand you need to take a break and celebrate.

Log in to your account, go to your account dashboard, then click Manage Plan, a new page will pop up (Powered by Stripe), follow the step by step instructions to cancel your subscription.

a. Your access will be revoked at the end of your subscription period (same as Netflix)

b. Your solutions will also be removed, if you would like to keep a copy of all your queries, please contact us

If you want to permanently delete your account, simply shoot us a message with your username and email, and we will delete your account within 24 business hours.

Since we have already provided a FREE tier for all of our users to practice all of our coding questions. Generally speaking, we do not refund you unless it's absolutely necessary. 

But of course, we welcome any suggestions, please feel free to let us know if you have any.

Have more questions? Please feel free to contact us, we read and respond to every email you send us, just give us some time. 😃