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A curated list of 230 SQL & Python coding questions and solutions.

Available engines: PostgreSQL, MySQL, Python and R.

118. Daily bookings in the US

hard airbnb

  • Write a query to return the daily number of bookings (reservations) in July 2021 in the US.
  • If there is no booking, return 0.

Table 1: bookings

When someone makes a reservation on Airbnb.


  col_name    | col_type
--------------+-------------------
date          | date
booking_id    | bigint
listing_id    | bigint


Table 2: dates

Calendar dates from 01/01/2019 to 12/31/2025.


 col_name | col_type
----------+----------
 year     | smallint
 month    | smallint
 date     | date

Table 3: listings

When a new Airbnb listing is created in a country ('US', 'UK', 'JP', 'CA', 'AU', 'DE')


  col_name    | col_type
--------------+-------------------
listing_id    | bigint
country       | varchar(2)
created_dt    | date


Solution postgres

WITH us_bookings AS (
    SELECT
        B.date,
        COUNT(booking_id) AS num_bookings
    FROM bookings B	
    INNER JOIN listings L
    ON L.listing_id = B.listing_id
    AND L.country = 'US'	
    GROUP BY date	
)
SELECT 
    D.date, 
    CASE WHEN B.num_bookings IS NULL THEN 0 ELSE B.num_bookings END AS num_bookings
FROM dates D
LEFT JOIN us_bookings B
ON B.date = D.date
WHERE D.date >= '2021-07-01'
AND D.date < '2021-08-01'
ORDER BY D.date;
    

Explanation

This query is designed to count the number of bookings made in US listings for each date within the specified time range. It accomplishes this by first selecting all bookings made in US listings and grouping them by date. This subquery is named "us_bookings".

The main query then selects all dates within the specified time range and left joins them with the "us_bookings" subquery on the date field. This is done to ensure that all dates within the specified range are included in the result, even if there were no bookings made on that date.

The final select statement returns the date and the number of bookings made on that date, with a case statement to handle cases where there were no bookings made on a particular date.

The result is ordered by date.

Expected results

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Hi Leon, I hope you're doing well. I wanted to reach out and say thanks for your interview prepβ€”it really helped me land a position at Amazon. It's been a fulfilling experience!

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I got a job offer at Amazon as a Data Engineer! I worked with Leon and he was super helpful to me at every stage of the process of getting a new role! He helped me strategize a study plan for technical interviews, shared knowledge of what different data roles are like and which one I would like, and evaluate and negotiate what is important to me in the offer!

The best thing that I learned working with Leon is how to spend my time effectively in the interview process. Interviewing is not one of my strengths, but Leon empowered me to get to my goals!

Leon is really smart and has years of experience in Data, and knows the hiring process inside and out. It is so worth it to be able to learn from his experience! I have nothing but gratitude!

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Your advice and coaching are extremely helpful for preparations for the interviews. The timeline and pacing you helped plan worked out very well.

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Just wanted to share, I accepted an offer from Apple as a Data Engineer and wanted to offer a huge thank you to both of you guys.

Even though I only shared a time with you both I benefited a lot from your platform and your mock interviews.

Some of the questions you gave during our Data Modeling with you Mike were presented verbatim.

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Shout out to Leon Wei from sqlpad.io, recommending him if you are searching for great advises and mentorship for your #datasciences career.

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πŸ’¬ SQLPad helped me gain mastery of all the core SQL concepts in a structured manner with a thoughtfully designed business schema. The associated articles and videos are also very well produced and helped me get a better understanding of translating business questions to actual SQL code. Highly recommended for anyone looking to deepen their SQL skills and those applying for data analytics roles.

Shoutout to Leon Wei from instamentor.com.

Please do reach out to him if you're looking for career advice, general consulting, or resume feedback. I found his services extremely valuable.

And while you're at it, please do check out SQLPad if you're looking to refresh your SQL concepts.

sqlpad.io seems more intuitive and lightweight where some of the other sites try to do too much and they're a bit cumbersome to use.

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I signed up for your emails, was going through the mini courses, using the Playground and I got more benefit out of all of that in two weeks than I did at the other websites. Glad I found sqlpad.io.

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Thank you so much for your help and support this year. I am pleased to say I received two offers Software Engineer 3 at Walmart and a Senior Machine learning engineer position at JLR based out of the UK.

Simply put, SQLPad is not the cheapest SQL interview prep site, but its high quality SQL interview questions makes it worth every penny. You can't practice anywhere else of such well thought and carefully designed business context oriented list of SQL coding questions.

I nailed my SQL interviews and received multiple Data Scientists offers that potentially double my currently salary, which makes the few hundreds dollars purchase of SQLPad lifetime bundle a steal.

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With sqlpad.io, I can focus on questions by skill level or topic, and the number of questions available is a reasonable amount to study for interview prep.

In addition, I got mentoring sessions with Leon and I was impressed at how helpful it was. Leon was calm and knowledgeable, which helped my nerves over my up-coming interview.

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I have been relearning SQL and sqlpad has been a great resource. There is a good ratio of a new concept to practice questions! Highly recommended.

πŸ’¬ "I signed up on SQLPad and was pleasantly surprised when Leon helped me out personally on the website.

In two mock interviews with him, he was meticulously prepared and very professional.

Throughout the interview he provided fantastic feedback and frameworks on how to be better prepared for both technical and behavioural aspects of an interview. He is also a very positive and friendly person and I enjoyed all my interactions with him.

πŸ’¬ SQLPad is the best website I have used for practicing SQL. The databases and practice problems resemble real-world data and daily tasks in a data scientist / data analyst role. I landed a new job as a data scientist at a tech company after completing all the practice problems. I would definitely recommend SQLPad to all who would like to improve their SQL skills and prepare for technical interviews.

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Commonly asked questions

Picture this: you're scrolling through LinkedIn when suddenly, a message pops up, "Do you have time for a chat this week?"

It's a recruiter from none other than Google, your dream company. Your heart skips a beat.

You manage to impress in the initial call with the recruter, and then you're onto the technical round 3 days later.

This is it, your moment to shine.

But the task is to write a SQL query to identify top search terms by country.

Despite being a SQL wizard at work, without your trusted code editor, the familiar auto-complete, and the usual business context, you find yourself floundering.

Minutes tick by, and you can sense the interviewer's interest waning as he started looking at his phone.

You cobble together a semi-optimized code and present it, only to be met with a quick glance and a glance at the watch.

"We have 5 minutes left, do you have any questions for me?" he ask, signaling the end.

The next day, an email arrives from the recruiter.

It's polite, regretful—they've decided not to move forward. Those words sting, leaving you wondering, "What went wrong?"

Does this scenario sound painfully familiar?

It's a story we've heard too many times, and it's exactly why SQLPad was created.

SQLPad is more than just a learning platform; it's your end-to-end career launchpad.

Beyond our expertly designed courses and interactive coding challenges in SQL, Python, R, and machine learning, SQLPad brings an innovative suite of AI tools to elevate your job application process.

From crafting the perfect resume to penning compelling cover letters, our AI-driven assistance ensures your first impression is as impressive as your skills.

But we don't stop there. Our AI-powered mock interview feature simulates real-life interviews with hiring managers, allowing you to practice your responses, refine your delivery, and gain valuable feedback in real-time.

This unique tool is designed to build your confidence, ensuring you can articulate your value and tackle any question with poise and assurance.

Ready to transform your job application from a point of stress to a point of strength?

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Save up to 70%, and there is NO RECURRING CHARGES

Check it out: SQLPad Annual Pass here.

Certainly, many customers have had their subscriptions reimbursed by their employers, enabling them to study SQL and Python without incurring personal costs.

Most companies allocate a specific annual budget (thousands of dollars) for employee training and learning, both for tax advantages and other benefits.

Furthermore, numerous companies automatically approve expenses under $100 without the need for managerial consent.

Here is a sample invoice you can use to get reimbursed.

sample sqlpad invoice for reimbursement

Need some help convincing your manager? Here is a draft email for your reference:

Dear Elon,

I would like to request your support in reimbursing my SQL and Python training at SQLPad.io.

By learning SQL and Python, I can improve my productivity and work more efficiently in managing and analyzing large datasets.

I have reviewed the course materials and believe that the interactive tutorials and real-world projects would provide a comprehensive learning experience. This investment will benefit not only me but also our team and the organization as a whole. Thank you for considering my request.

Best regards,

Steve

You are given 10 FREE SQL/Python coding questions to try out, after that, your account will be locked.

Upgrade to a paid plan will unlock it immediately and also give you access to all questions and solutions, as well as the SQL, Python, Machine Learning Courses.

On average, it takes about 3-4 months to finish all the coding questions.

Since we have already provided you a FREE tier to practice a limited set of questions, we cannot refund you once your subscription starts.

Cancellation and Refund Policy

1. Monthly/Quarterly subscriptions
Once your subscription starts, you CAN NOT get a refund for any payments you've already made.
If you don't want to subscribe anymore, cancel your subscription.

You will have access to the subscription until the end of the period you have paid for.

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

To cancel your subscription, simply:

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Leon Wei, SQLPad founder Hi, my name is Leon Wei. Most recently, I was a senior manager of machine learning at ο£Ώ Apple. I am currently building an AI data analytics tool: skills.ai while running sqlpad.io. You can read more about me at my linkedin profile.

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

Before that, I lead a team of data scientists and engineers at Chegg, helping students or young professionals to improve their performance at school or work.

I also worked as a research scientist at Amazon, focusing on building a large-scale real-time pricing optimization engine for their online retail business. 

We created the cracking the SQL interview for the data scientist course with a beginner's mindset, and we assume our audience has 0 experience with SQL.

That's why the first 10 questions may seem to be very easy for experienced users, but you will find more and more challenging and tricky hard level questions as you move along to later questions.

The goal of sqlpad is to help people in data analytics/science quickly learn or refresh their SQL, Python/Pandas skills.
It could be for a job interview or be more productive at work.

We offer 20% off for students, educational institutions and startupsEmail us (hello at sqlpad dot io) from your .edu or work email to request a discount code.

We do not offer any other discounts.

Unfortunately NO.

We have designed and created the entire SQL course and curated all 200 SQL coding questions single-handedly, which took us more than 3000 hours of hard work, therefore you are not allowed to use any of those questions outside of this website, without our written permission.

If you represent a company and are interested to acquire a license to use our coding questions and solutions, please feel free to contact us.

From sqlpad's founder Leon wei.

After launching the Cracking the SQL Interview for Data Scientists course, I realized the need for my students to easily follow the course and practice SQL coding problems.

Since I can't find a solution online, I built SQLPad.

All the SQL exercises can be practiced in a browser, and my students don't have to waste hours configuring the database on their own computers.

As of March 21, 2023.

Those are the major version of our online playground.

PostgreSQL: 13.9

MySQL: 8.0

Python: 3.10

Pandas: 1.5

 

SQLPad is the Leetcode alternative that makes learning and mastering SQL for data analytics professionals instead of software engineers. It creates a complete SQL mastery path for all data scientists.


πŸ§‘‍πŸ’» Our approach to tackling a SQL interview is to first teach you SQL fundamentals with the cracking the SQL Interview for data scientists course before asking you to jump into the coding war.

πŸ“Ή More than 20 video lectures to learn SQL fundamentals and tips and tricks help you understand what a SQL interview is about.

πŸ‹οΈ‍♀️ After you grasp the basics, you then jump into SQL coding questions that challenge you and make sure you mastered those SQL fundamental concepts.

😱 Leetcode jumps directly into some super long, hard-to-understand SQL coding questions that may frustrate you, especially if you just started.

πŸ“ˆ Instead, SQLPad has a more gradual, more user-friendly learning experience for all SQL skill levels.

🍿 The first 80 SQL coding questions are based on the same Movie rental database, which resembles a real-world commerce business.

πŸ€— From easy-level questions to challenging ones, you gradually build up a good understanding of the database schema by practicing those questions.

πŸ‘©‍πŸ’» And by the time you are to solve some of the most advanced SQL interview questions (#58-80), such as WINDOW functions and LAG/Lead, you are already familiar with those tables. You can jump right into the coding environment after learning the concepts.

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.

(Update 01/26, 2022, we now also support MySQL and Python, more info.)

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

Its query syntax is similar to other databases such as MySQL or Microsoft SQL Server, one of the major differences 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 email us if you have any questions.

Yes, we now support dark mode!

To switch between light and dark mode, simply click the toggle button located at the top right of the navbar.

Yes, you totally can.

SQL interview questions and  SQL Playground work in all modern web browsers, including Chrome, Safari, Firefox in ο£Ώ Mac, Windows, Linux  and other operating systems.

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 for Data Scientists Course can follow along.

I have been gradually adding more solutions (optimized) and will indicate if that solution is for more advanced users.

If you have a better solution than mine, please feel free to let me know!

 

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

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

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