20 Advanced SQL Interview Questions (With Answers!)

Looking to ace your next SQL interview with confidence? Test yourself with these 20 advanced SQL interview questions, each accompanied by detailed answers to help you impress recruiters and land that dream job. These questions cover everything from complex queries to data manipulation and optimization techniques, and equip you with the knowledge you need to excel in any SQL interview scenario.

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Advanced SQL Interview Questions

Interview questions that test practical and conceptual SQL skills are to test the candidate's ability to translate knowledge into effective solutions. The following questions are 20 SQL interview questions pulled from REAL interviews, that helped managers assess candidates' SQL skills.

While we have included sample answers for each question, remember that these are not the only correct answers. There are often multiple ways to achieve the same goal. Focus on writing clean and functional code.

1. Repeated Payments [Stripe SQL Interview Question]

Stripe asked this tricky SQL interview question, about identifying any payments made at the same merchant with the same credit card for the same amount within 10 minutes of each other and reporting the count of such repeated payments.

Input/Output Data

Example Input:

Example Output:

Solution:

You can solve this Stripe SQL problem interactively and get a full solution explanation:

2. Describe how recursive queries work in SQL. Provide an example scenario where a recursive query would be beneficial.

Recursive queries in SQL enable iteration over hierarchical data structures like organizational charts, bill-of-materials, or nested comments. For example, you could use a recursive query to traverse a hierarchical data structure and retrieve all descendants of a specific node.

3. Median Google Search Frequency [Google SQL Interview Question]

Google’s Marketing Team needed to add a simple statistic to their upcoming Superbowl Ad: the median number of searches made per year. You were given a summary table that tells you the number of searches made last year, write a query to report the median searches made per user.

Table:

Input/Output Data:

Example Input:

Example Output:

Solution:

You can solve this Google SQL problem interactively, and get the full solution explanation.

Enjoyed this one? p.s. here's more Google SQL Interview Questions to practice!

4. What are ACID properties in SQL?

ACID properties are:

• Atomicity: Changes in the data must be like a single operation.
• Consistency: The data must be consistent before and after the transaction.
• Isolation: Multiple transactions can be done without any hindrance.
• Durability: Transaction gets successful in case of system failures.

5. Monthly Merchant Balance [Visa SQL Interview Question]

Say you have access to all the transactions for a given merchant account. Write a query to print the cumulative balance of the merchant account at the end of each day, with the total balance reset back to zero at the end of the month. Output the transaction date and cumulative balance.

Table:

Input/Output Data:

Example Input:

Example Output:

Solution:

You can solve this Visa SQL problem interactively, and get the full solution explanation.

6. Explain the concept of correlated subqueries in SQL. Can you provide an example of how you can use them?

Correlated subqueries are executed once for each row of the outer query and depend on the outer query for results. For example, you could use a correlated subquery to retrieve data from one table according to conditions in a separate table.

7. Server Utilization Time [Amazon SQL Interview Question]

Fleets of servers power Amazon Web Services (AWS). Senior management has requested data-driven solutions to optimize server usage.

Write a query that calculates the total time that the fleet of servers was running. The output should be in units of full days.

Assumptions:

• Each server might start and stop several times.
• The total time in which the server fleet is running can be calculated as the sum of each server's uptime.

Table:

Input/Output Data:

Example Input:

Example Output:

Solution:

You can solve this Amazon SQL problem interactively, and get the full solution explanation.

Looking for more Amazon SQL Interview Questions? Check out our Amazon SQL Interview Guide for access to even more questions and solutions!

8. Explain how indexing works in a relational database. What are the benefits and potential drawbacks of indexing?

Indexing in a relational database involves creating data structures (indexes) that allow for faster data retrieval by providing direct access to rows based on the indexed columns. The benefits of indexing include improved query performance and faster data retrieval, especially for large datasets. However, indexing can also introduce overhead during data modification operations (such as INSERT, UPDATE, and DELETE) and increased storage requirements.

9. Uniquely Staffed Consultants [Accenture SQL Interview Questions]

As a Data Analyst on the People Operations team at Accenture, you are tasked with understanding how many consultants are staffed to each client, and how many consultants are exclusively staffed to a single client.

Write a query that displays the outputs of client name and the number of uniquely and exclusively staffed consultants ordered by client name.

Table:

Input/Output Data:

Example Input:

Table:

Example Input:

Example Output:

Solution:

You can solve this Accenture SQL problem interactively, and get the full solution explanation.

10. Compare and contrast stored procedures and user-defined functions in SQL. When would you choose to use one over the other?

Stored procedures and user-defined functions (UDFs) are both reusable pieces of code in SQL, but they have some key differences. Stored procedures can perform DML operations and transaction management, while UDFs are limited to returning values and cannot perform DML operations. Stored procedures are typically used for procedural tasks and complex business logic, while UDFs are often used for calculations and data transformations.

11. Event Friends Recommendation [Facebook SQL Interview Questions]

Facebook wants to recommend new friends to people who show interest in attending 2 or more of the same private events.

Sort your results in order of user_a_id and user_b_id (refer to the Example Output below). Table:

Each row of this table indicates the status of the friendship between user_a_id and user_b_id.

Input/Output Data:

Example Input:

Table:

Example Input:

Example Output:

Users 222 and 333 who are not friends have shown interest in attending 2 or more of the same private events.

Solution:

You can solve this Facebook SQL problem interactively, and get the full solution explanation.

Looking for more Facebook SQL Interview Questions? Check out our Facebook SQL Interview Guide for access to even more questions and solutions!

12. Explain the purpose of the MERGE statement in SQL. Provide an example.

The MERGE statement performs INSERT, UPDATE, or DELETE operations on a target table based on the results of a join with a source table.

Example:

13. 3-Topping Pizzas [McKinsey SQL Interview Question]

You’re a consultant for a major pizza chain that will be running a promotion where all 3-topping pizzas will be sold for a fixed price, and are trying to understand the costs involved.

Given a list of pizza toppings, consider all the possible 3-topping pizzas, and print out the total cost of those 3 toppings. Sort the results with the highest total cost on the top followed by pizza toppings in ascending order.

Break ties by listing the ingredients in alphabetical order, starting from the first ingredient, followed by the second and third.

** Table:**

Input/Output Data:

Example Input:

Example Output:

Solution:

You can solve this McKinsey SQL problem interactively AND see alternate solutions, and get the full solution explanation.

14. Describe the differences between clustered and non-clustered indexes in SQL.

A clustered index determines the physical order of data in a table, while a non-clustered index does not. A table can have only one clustered index but multiple non-clustered indexes.

15. Follow-Up Airpod Percentage [Apple SQL Interview Questions]

The Apple retention team needs your help to investigate buying patterns. Write a query to determine the percentage of buyers who bought AirPods directly after they bought iPhones. Round your answer to a percentage (i.e. 20 for 20%, 50 for 50) with no decimals.

Table:

Input/Output Data:

Example Input:

Example Output:

Solution:

You can solve this Apple SQL problem interactively, and get the full solution explanation.

Looking for more Apple SQL Interview Questions? Check out our Apple SQL Interview Guide for access to even more questions and solutions!

16. What is the purpose of the LAG and LEAD functions in SQL? Provide an example.

The LAG and LEAD functions allow accessing data from previous or subsequent rows in a result set.

Example:

17. Marketing Touch Streak [Snowflake SQL Interview Question]

As a Data Analyst on Snowflake's Marketing Analytics team, your objective is to analyze customer relationship management (CRM) data and identify contacts that satisfy two conditions:

1. Contacts who had a marketing touch for three or more consecutive weeks.
2. Contacts who had at least one marketing touch of the type 'trial_request'.

Marketing touches, also known as touch points, represent the interactions or points of contact between a brand and its customers.

Your goal is to generate a list of email addresses for these contacts.

Table:

Input/Output Data:

Example Input:

Table:

Example Input:

Example Output:

Solution:

You can solve this Snowflake SQL problem interactively, and get the full solution explanation.

Looking for more Snowflake SQL Interview Questions? Check out these 10 Snowflake SQL Interview Questions for access to even more questions and solutions!

18. Explain the purpose of the TRUNCATE statement in SQL. How is it different from DELETE?

The TRUNCATE statement removes all rows from a table but does not log individual row deletions, making it faster than DELETE, which removes rows one by one and logs each deletion. However, TRUNCATE cannot be rolled back, and it resets identity columns.

19. Bad Delivery Rate [DoorDash SQL Interview Question]

The Growth Team at DoorDash wants to ensure that new users, who make orders within their first 14 days on the platform, have a positive experience. However, they have noticed several issues with deliveries that result in a bad experience.

These issues include:

• Orders being completed incorrectly, with missing items or wrong orders.
• Orders not being received due to incorrect addresses or drop-off spots.
• Orders being delivered late, with the actual delivery time being 30 minutes later than the order placement time. Note that the is automatically set to 30 minutes after the .

Write a query that calculates the bad experience rate for new users who signed up in June 2022 during their first 14 days on the platform. The output should include the percentage of bad experiences, rounded to 2 decimal places. Table:

Input/Output Data:

Example Input:

Table:

Example Input:

Table:

Example Input:

Example Output:

Solution:

You can solve this DoorDash SQL problem interactively, and get the full solution explanation.

Looking for more DoorDash SQL Interview Questions? Check out these 8 DoorDash SQL Interview Questions for access to even more questions and solutions!

20. Describe the difference between a view and a materialized view in SQL.

A view is a virtual table based on the result of a SELECT query, while a materialized view is a physical copy of the result set of a query that is stored and updated periodically.

SQL Interview Tips

The best way to prepare for the SQL interview is to practice, practice, practice. Besides solving the earlier SQL interview questions, you should also solve the 200+ SQL coding questions which come from companies like FAANG tech companies and tech startups.

Each exercise has hints to guide you, step-by-step solutions and best of all, there is an interactive coding environment so you can right online code up your SQL query and have it checked. However, if your SQL skills are weak, forget about going right into solving questions – refresh your SQL knowledge with this DataLemur SQL Tutorial.

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