SQL Interview Questions: A Guide for Data Analysts

SQL, which stands for Structured Query Language, ranks amongst the essential skills for data analysts. As you prepare to interview for data analyst jobs, you can expect SQL to come up during the interview process.

With this guide, you’ll learn more about SQL technical screenings, what type of screening you might encounter, and some common questions you may be asked at an interview. You’ll also find some example questions, a step-by-step guide for writing SQL code during your interview, and tips for success. Let’s get started.  

What to expect from SQL technical screenings

Data analysts use SQL to communicate with relational databases to access, clean, and analyse data. Since it’s such a critical skill, it’s common for data analyst interviews to include a SQL technical screening. 

The SQL segment of the interview tasks you with solving real-world problems. While you may be asked some definitional questions typical of a standard interview, the interviewer wants to verify that you can use SQL, not just talk about it. 

These screenings typically take one of three forms:

1. Whiteboard test: The whiteboard interview is the most common type of SQL screening. In this type of screening, you’re given a whiteboard and marker to write your queries by hand. Since you won’t have a computer alerting you to any syntax or logical errors in your code, this is more about demonstrating that you can think through a problem and know the right SQL concepts to solve it. 

2. Live coding: With this type of screening, you’ll be given SQL problems to solve in a live coding environment. This allows you to run your queries and cheque your work as you go, but since you’re running your code, syntax will matter. Since different databases use different tools, this type of screening isn’t as common as the whiteboard screening.

3. Take-home assignment: With this less-common screening technique, you’ll get a problem or series of problems to take home and solve within a given time. This lets you write your queries in the comfort of your home without the pressure of an interviewer looking over your shoulder. On the other hand, the coding challenges are often more complex.

Types of SQL interview questions for data analysts

Not only do technical screenings take one of three formats, the questions you’ll typically answer during this portion of the video fall into three broad categories. We’ve arranged them here from simple to complex. Generally speaking, the easier, definitional questions will be fewer and less important than the live coding questions—something to keep in mind as you prepare.

1. Define a SQL term

If you’re interviewing for a data analyst role, chances are you know what SQL is (and your interviewer assumes you know this). It’s possible interviewers will ask what SQL is, but it’s more likely you’ll need to explain more technical concepts in SQL, the difference between two or more related concepts, or how to use a concept. This is not an exhaustive list, but here are some examples of terms you should be ready to explain:

• Trigger

• Index

• Cursor

• Constraints

• ETL (Extract, transform and load)

• Primary key, foreign key, and unique key

• Normalisation vs. denormalisation

• RDBMS vs. DBMS

• Clustered vs. non-clustered index

Forms definitional questions may take:

1. What is the purpose of an index in a table? Explain the different types.

2. What are the types of joins in SQL?

3. What is the difference between DROP, TRUNCATE, and DELETE statements?

4. How do you use a cursor?

5. What is the difference between a HAVING clause and a WHERE clause?

2. Questions about a query

This second category of questions gives you an SQL query and asks you a question about it. This tests your ability to read, interpret, analyse, and debug code written by others.

Forms query analysis questions may take:

Given a query:

1. Put the clauses in order by how SQL would run them.

2. Identify the error and correct it.

3. Predict what the query will return.

4. Explain what problem the query solves.

3. Write a query

Many questions commonly associated with the SQL technical screening ask you to solve a given problem by writing out a query in SQL. You’ll typically get one or more tables and instructions to write one or more queries to retrieve, edit, or remove data from those tables.

The difficulty of questions will likely vary based on the company and the role (entry-level vs. advanced). In general, you should be comfortable writing queries using the following concepts, statements, and clauses:

• Categorisation, aggregation, and ratio (CASE, COUNT, or SUM, numerator and denominator)

• Joining two tables (JOIN inner vs. left or right)

• Modifying a database (INSERT, UPDATE, and DELETE)

• Comparison operators (Less than, greater than, equal to)

• Organising data (ORDER BY, GROUP BY, HAVING)

• Subqueries 

Forms query-writing questions may take:

Given a table or tables with a few sample rows:

1. List the three stores with the highest number of customer transactions.

2. Extract employee IDs for all employees who earned a three or higher on their last performance review.

3. Calculate the average monthly sales by product displayed in descending order.

4. Find and remove duplicates in the table without creating another table.

5. Identify the common records between two tables.

6-step strategy for your SQL interview

Sometimes the best way to calm nerves before an interview is to walk into the screening with a clear action plan. No matter what type of query interviewers ask you to write, this six-step process can help you organise your thoughts and guide you to a solution, even when you’re feeling nervous.

1. Restate the question to make sure you understand what you must do.

2. Explore the data by asking questions. For example: What data type is in each column? Do any columns contain unique data, such as user ID? 

3. Identify the columns needed to solve the problem. This helps you focus on the data that matters so you’re not distracted by the data irrelevant to the query.

4. Think about how your answer should look. Are you looking for a single value or a list? Will the answer come from a calculation? If so, is it a float or an integer? Do you need to account for this in your code?

5. Write your code one step at a time. It can help to outline your approach first. By writing down the steps you plan to take, you’ll have a clear outline once you start writing your query (and you’ll give the interviewer a chance to correct you if there’s an issue with your approach).

Then, code in increments, taking one step of your outline at a time. After you’re happy with your code for the first step, build onto that code with the second step. 

6. Explain your solution as a whole. If there’s a more efficient way you could have written your code—using subqueries, for example—explain that. And remember to answer the original question.

SQL interview tips for success

In addition to the process above, here are some tips to keep in mind when you’re in your SQL interview.

• Talk through your process out loud. Your interviewer may or may not know SQL themselves, so explain each step’s what, how, and why.

• Include written comments explaining what you intend each step of your query to accomplish. This can help you keep track of where you are in the problem and make your code easier to understand. If you’re coding in a live environment, you can type comments using a double hash (--). On a whiteboard, write your comments off to the side.

• Use correct formatting. While your problem-solving ability is more important than precise syntax, you can avoid confusing the interviewer (and yourself) by keeping your hand-written code organised.

• Embrace the awkwardness. It’s okay if the room is silent while you think through a problem. As you’re thinking aloud, you may re-start sentences with a better way to explain something. That’s okay too.

Next steps

Thinking about a career as a data analyst? Start building job-ready skills like SQL in less than six months from experts at Google with the Google Data Analytics Professional Certificate on Coursera.