SQL Interview Questions (and How To Answer Them)

A quick look at job postings for data analysts will reveal how knowledge of SQL is listed as a requirement for most of them. As interviews tend to be a stressful experience, this blog post aims to help demystify different aspects of SQL interviews, and how you can best prepare ahead of time to ace them. We’ll take a look at specific types of SQL interview questions and share some tips on how to study for them!

  1. What should I expect from an SQL interview?
  2. Types of SQL interview questions for data analysts
  3. How to ace your SQL interview
  4. Summary and next steps
Related watching: What tools do data analysts use?

1. What should I expect from an SQL interview?

Whiteboard test

A whiteboard test is one where you solve the technical coding challenge in real time, in front of an audience, while either writing code or conceptual diagrams on a whiteboard. These typically aren’t just evaluating your technical acumen but your non-technical skills as well, as interviewers look for how you communicate your solutions, think through the challenge in a systematic way, and handle questions on the fly. 

Oftentimes, interviewers are not looking for you to know how to write a perfect SQL query; being able to explain how it would work through pseudo code can be good enough. The emphasis is less on whether you remember the SQL syntax perfectly and more on your knowledge of how SQL works and whether you would know how to retrieve the required data from a database. 

Related watching: What is a whiteboard challenge?

Live coding

Most companies will introduce a live coding test (or two) as part of the technical screen. This typically involves an in-person or virtual exercise completed live and with the interviewer(s) watching how you tackle the question and code the solution. Whether in-person or remotely, you will generally do this by using your preferred code editor such as PyCharm or Visual Studio Code

As you can type your solutions on a computer, the bar is a little higher than in a whiteboard test. You’ll be expected to have stronger knowledge of SQL syntax since you can run your queries live to verify if they returned the correct tables, or if any syntax errors pop up. 

Take-home task

The take-home assignment, a staple of data science interviews, is less commonly seen in SQL interviews. That said, it can come up in companies that value the opportunity to look more in-depth at your SQL skills within the context of a larger analytical question. The focus here is less on your ability to solve a niche, abstract SQL question, and more on how you use SQL with the end goal of extracting business insights. 

Companies typically present you with synthetic data that matches the distribution of data they deal with daily and a few business questions to answer. At a minimum, the solution should include clean, optimized SQL syntax, since you have ample time to work on it and run queries to check for errors. You may be asked to present your results in a Jupyter notebook or through a short slide deck presentation.

2. Types of SQL interview questions for data analysts

The types of SQL interview questions that data analysts get generally fall into these three categories of increasing difficulty.

SQL interview questions: Defining SQL terms

Sometimes, the initial interview screen will involve conceptual questions to test whether you have sufficient knowledge of SQL and how it relates to databases. This may involve questions as basic as listing the types of joins in SQL, what a common table expression is (CTE), how window functions work, what an index is, or trickier questions like the difference between similar clauses (e.g. having versus where). 

Try not to overlook basic questions like these when preparing for SQL interviews, as companies want to ensure that your knowledge of SQL is not surface-level or overly focused on writing queries without understanding how it fits into the world of database management. 

SQL interview questions: Clarifying SQL queries

The next level of difficulty involves testing your knowledge of how SQL queries actually work. Interviewers will present you with a pre-written query and ask you a few questions related to its purpose and structure. 

They may ask you to find any errors and ask questions similar to the following: describe how you would rewrite it correctly? How would you modify the query to return a result that would better illuminate more profitable lines of business? What SQL syntax would you use to transform the data type for a certain column? A more challenging question this category might ask you to predict what the query returns, especially if the query is more complex with multiple group bys and aggregations.

Related reading: SQL Cheatsheet: Learn Your First 8 Commands

SQL interview questions: Writing SQL queries

The hardest level involves writing a query for a stated problem. You will need to have a strong grasp of the basics for this stage, such as knowing the correct order for SQL statements, and being fluent in using common keywords like select, from, where, order by, and as. You will also need to know how to use aggregation functions to count or find minimum and maximum values, grouped by a category or date, and join as many tables as necessary to retrieve the correct subset of data. 

To show off your SQL skills, learn how to use more advanced techniques such as window functions. These are functions that create a subset of data (partition) over which you can run aggregations. Other techniques include common table expressions (creation of tables for later use) and subqueries (queries within queries). 

Another way to earn extra points here is to pay attention to query performance. This simply means optimizing the query to run quickly when dealing with very large datasets. Even if the sample dataset you’re presented with is just a small, toy dataset, it’s worth signaling your awareness of this concept by including it in the syntax. Ensure that your query includes limits or indices, and mention how you would run expensive queries on a schedule to reduce computation cost. 

3. How to ace your SQL interview

The best way to ace an interview is to prepare, prepare, prepare. This is especially true for the SQL interview, where so much of it focuses on technical knowledge that cannot be attained any way else but through consistent practice and familiarity.  

With whiteboard tests, it helps to replicate the interview environment by purchasing your own whiteboard to practice on. Ask a friend to roleplay as the interviewer so you can get comfortable writing code without being allowed to use technological aids like Github CoPilot or searching StackOverflow for answers, all in front of an audience.

Brush up on your SQL syntax by practicing on as many questions as you can. Live coding questions are especially tough as not many are used to being watched while coding. To help with the nerves, it’s critical to get exposure to as many types of SQL questions as possible. These questions vary greatly in complexity and can be tricky to prepare for. Many aspiring programmers turn to online programming question banks like LeetCode and HackerRank, which have a wide range of SQL coding challenges. 

Other users often post their solutions online, and reverse-engineering better solutions is a great teaching tool as well. The great thing about practicing on these well-regarded platforms is that many companies draw their questions from there, so you’ll likely come across questions that you have worked on previously. 

With take home tests, you can exceed expectations and really stand out as a candidate by developing strong verbal and written communication skills. This means including comments and section headers in the Jupyter notebook solution, or learning how to connect the technical results to the business questions they posed. 

As most candidates focus on developing strong technical skills, you’ll make an impression if you always make sure that the SQL code is not only clean and optimized, but also situated within the larger and more important context of how the results impact business outcomes. 

Related reading: The common data analytics interview questions you’ll be asked

4. Summary and next steps

Interviews are tough, and tech interviews are known for being especially grueling, with their many rounds of technical interviews that come in different formats that make it hard to prepare well for. We’ve given you a broad overview of what you can expect to encounter in a typical SQL interview, the types of SQL interview questions you might receive, and our best strategies in preparing them, whether it’s a live whiteboard test or a remote take-home assignment. 

Always remember to draw the technical results back to the big picture of business outcomes, as companies appreciate that you understand SQL is ultimately one of many available tools used by analysts to drive strategic impact. 

Interested in learning more about SQL and the world of data analytics as a whole? Check out this list of SQL certifications to brush up on your knowledge. Or why not try out our free, self-paced data analytics short course?

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