How to Answer SQL Interview Questions

In this lesson, we'll cover how to structure your SQL interview answers. We've consulted with several data analysts and SQL experts to develop our below framework.

1. Understand the Data

Before starting to write any queries, review and understand the table and data types involved in the problem.

Example: "Before I start writing any queries, I want to make sure I understand the table and the data types involved. From my analysis of the table, I can see that it has several columns such as ID, name, date, and so on. Could you share a bit more about the way the data in the date field is represented? Also, does the name field typically include both first and last name?"

2. Ask Clarifying Questions

Often, the SQL interview question may not provide all the information you need to solve the problem. Asking clarifying questions ensures you understand not just the table but also the question and goals of the question. Additionally, if certain terms are not defined, be sure to ask the interviewer to define them and map them to the data.

Example: "Just to clarify, when you say 'revenue,' are you referring to net revenue or gross revenue? Also, could you define the term 'churn' as it pertains to this particular business? These clarifications will help me tailor my query to the specific problem at hand."

3. Discuss Approach

After understanding the problem and the data, confirm the overall plan with the interviewer and map out the approach. This is a great opportunity to align with your interviewer on the approach so that your interviewer and you are on the same page. Take feedback and check in with your interviewer before you code the answer.

If helpful, you can write pseudocode for the query to make sure that you have a clear idea of how to solve the problem before you start writing the actual code.

Example: "Based on my analysis of the data, I plan to use a subquery to filter out the relevant data before using a join statement to combine it with the other table. This should give me the results we're looking for in a reasonable amount of time. Does this approach make sense to you?"

4. Implement Code

With a plan in place, you can now enter the meat of the interview: implementing the SQL code. Be sure to verify and debug your code when writing it out.

Example: (While writing on the whiteboard) "I'm going to start by writing the SELECT statement to retrieve the relevant columns, followed by the FROM clause to specify the table. Then I'll add the WHERE clause to filter the data based on the criteria we discussed earlier. Finally, I'll use the GROUP BY clause to aggregate the data and get the results we're looking for."

5. Explain Code

Once you've implemented the code, explain your thought process and the logic behind your SQL query. Confirm that the output is as expected based on the approach you discussed with the interviewer.

Example: "So what I'm doing here is selecting the columns we need from the table and using a subquery to filter the data to only include the relevant transactions. Then I'm joining this subquery with the main table to get the desired results. This approach should be efficient and effective."

6. Discuss Results and Tradeoffs

After running the query, discuss the results and any takeaways you'd be looking for. Additionally, discuss any tradeoffs in implementation or outcome that might affect the effectiveness or efficiency of your query.

Example: "Based on the results we got, we can see that the churn rate for this particular cohort is higher than expected. While this is not ideal, it does give us an opportunity to identify areas for improvement and make changes accordingly. As for tradeoffs, using a subquery can be computationally expensive, so we'll want to make sure we're not overloading the system with too many queries at once.

Remember, this framework isn't one size fits all, but it's a helpful starting place when thinking about answering SQL interview questions!