Over the last 6 years I’ve been part of many job interviews – on both sides of the table. The most fun, but also the most feared, part of the process is the technical screening. In this article, I’ll show you three SQL test exercises that, in my experience, are quite typical in data analyst job interviews. (And hey, these are “sample” SQL interview questions but they are heavily based on reality!)
Before the tasks – What can you expect in an SQL technical screening?
There are two common ways an SQL tech screening can be done.
The simpler but less common way is that you get a computer, a data set and a task. While you are solving the task, the interviewers are watching and asking questions. A little trial and error is totally fine, as long as you can come up with the correct solution in a reasonable amount of time.
The other, more difficult (and by the way much more common) way is the whiteboard interview. In this case, you don’t get a computer. You have to solve the task and sketch up the code on a whiteboard. This means that you won’t get feedback (at least not from a computer) on whether you made a logical or a syntax error in your code. Of course, you can still solve the tasks by thinking iteratively (cracking the different SQL problems one by one), but you have to be very confident with your SQL skills.
Additionally, usually you have to solve the tasks on the fly. Maybe you will get 3-5 minutes of thinking time but that’s the maximum you can expect. The good news is that because of that you will get relatively simpler tasks. (See the difficulty level below!)
Note: there are other types of tech screening – like the take-home assignment – where you can prove that you can solve more complex coding challenges, too.
Here are three SQL interview questions that are really close to what I actually got or gave on data analyst/scientist job interviews!
Try to solve all of them as if they were whiteboard interviews!
In the second half of the article, I’ll show you the solutions, too!
SQL Interview Question #1
Let’s say you have two SQL tables:
authors dataset has 1M+ rows; here’s the first six rows:
books dataset also has 1M+ rows and here’s the first six:
Create an SQL query that shows the TOP 3 authors who sold the most books in total!
(Note: I got a very, very similar SQL interview question for a data scientist position at a very well-known Swedish IT company.)
SQL Interview Question #2
You work for a startup that makes an online presentation software. You have an event log that records every time a user inserted an image into a presentation. (One user can insert multiple images.) The
event_log SQL table looks like this:
…and it has over one billion rows.
Note: If the
event_date_time column’s format doesn’t look familiar, google “epoch timestamp”!
Write an SQL query to find out how many users inserted more than 1000 but less than 2000 images in their presentations!
(Note: I personally created and used this interview question to test data analysts when I was freelancing and my clients needed help in their hiring process.)
SQL Interview Question #3
You have two SQL tables! The first one is called
employees and it contains the employee names, the unique employee ids and the department names of a company. Sample:
The second one is named
salaries. It holds the same employee names and the same employee ids – and the salaries for each employee. Sample:
The company has 546 employees, so both tables have 546 rows.
Print every department where the average salary per employee is lower than $500!
(Note: I created this test question based on a real SQL interview question that I heard from a friend, who applied at one of the biggest social media companies (name starts with ‘F.’ :))
Solution of SQL Interview Question #1
The solution code is:
SELECT authors.author_name, SUM(books.sold_copies) AS sold_sum FROM authors JOIN books ON books.book_name = authors.book_name GROUP BY authors.author_name ORDER BY sold_sum DESC LIMIT 3;
And here is a short explanation:
1. First you have to initiate the
JOIN. I joined the two tables by using:
SELECT * FROM authors JOIN books ON books.book_name = authors.book_name;
2. After that, I used a
SUM() function with a
GROUP BY clause. This means that in the
SELECT statement I had to replace the
* with the
sold_copies columns. (It’s not mandatory to indicate from which table you are selecting the columns, but it’s worth it. That’s why I used
3. Eventually, I
ORDERed the results in
DESCending order. (Just for my convenience, I also renamed the
sum column to
sold_sum using the
AS sold_sum method in the
Solution of SQL Interview Question #2
The SQL query is:
SELECT COUNT(*) FROM (SELECT user_id, COUNT(event_date_time) AS image_per_user FROM event_log GROUP BY user_id) AS image_per_user WHERE image_per_user < 2000 AND image_per_user > 1000;
The trick in this task is that you had to use the
COUNT() function two times: first, you had to count the number of images per user, then the number of users (who fulfill the given condition). The easiest way to do that is to use a subquery.
- Write the inner query first! Run a simple
COUNT()function with a
GROUP BYclause on the
- Make sure that you create an alias for the subquery (
AS image_per_user). It’s a syntax requirement in SQL.
- Eventually, in an outer query, apply a
WHEREfilter and a
COUNT()function on the result of the subquery.
Solution of SQL Interview Question #3
SELECT department_name, AVG(salaries.salary) AS avg_salaries FROM employees JOIN salaries ON employees.employee_id = salaries.employee_id GROUP BY department_name HAVING AVG(salaries.salary) < 500;
Note: You can solve this task using a subquery, too – but in an interview situation the committee will like the above solution better.
1. First JOIN the two tables:
SELECT * FROM employees JOIN salaries ON employees.employee_id = salaries.employee_id
Watch out! Use the
employee_id column – not the
employee_name. (You can always have two John Does at a company, but the employee id is unique!)
2. Then use an
AVG() function with a
GROUP BY clause — and replace the
* with the appropriate columns. (Just like in the first task.)
3. And the last step is to use a
HAVING clause to filter by the result of the
AVG() function. (Remember:
WHERE is not good here because it would be initiated before the
Watch out: in the
HAVING line, you can’t refer to the alias – you have to use the whole function itself again!
Prepare for SQL tech screenings by practicing!
If you managed to solve all these questions properly, you are probably ready for a junior or even for a mid-level Data Analyst SQL technical screening.
If not, let me recommend my new online course: SQL for Aspiring Data Scientists (7-day online course) – where you can level up (or brush up) your SQL skills in only 7 days. When you finish the course, just come back to this article and I guarantee that you will be able to solve these questions!
And if you are just about to start with SQL, start with my SQL For Data Analysis series on the blog!
The hard part of these SQL interview questions is that they are abstract. The tasks say to “imagine the data sets” and show only a few lines of them. When you get an exercise like that, it helps a lot if you have seen similar datasets and solved similar problems before. I hope solving the tasks in this article will boost your confidence!
If you have questions or alternative solutions, don’t hesitate to leave a comment.
- If you want to learn more about how to become a data scientist, take my 50-minute video course: How to Become a Data Scientist. (It’s free!)
- Also check out my 6-week online course: The Junior Data Scientist’s First Month video course.