10 Twilio SQL Interview Questions (Updated 2024)

Updated on

March 1, 2024

At Twilio, SQL is used quite frequently for analyzing customer interaction data for text-message campaign optimization, and managing relational databases that power the voice, email, and telephony solutions Twilio has. Because of this, they frequently ask SQL questions during interviews for Data Analyst, Data Science, and BI jobs at Twilio.

So, to help you ace the Twilio SQL interview, we've curated 10 Twilio SQL interview questions – can you solve them?

Twilio SQL Interview

10 Twilio SQL Interview Questions

SQL Question 1: Identify the Top Users of Twilio's SMS Service

Let's say Twilio would like to identify its top users, focusing on the frequency of usage for their SMS service. The "power users" or "whale users" are defined as those who have sent out the most amount of SMS messages in the last 30 days.

Here are the example tables:

Example Input:
1John Doe
2Jane Smith
3Charlie Brown
4Lucy Van Pelt
Example Input:
101110/01/2022 00:00:00Sent
102210/03/2022 00:00:00Sent
103110/03/2022 00:00:00Sent
104310/07/2022 00:00:00Sent
105410/08/2022 00:00:00Sent
106110/09/2022 00:00:00Sent
107410/09/2022 00:00:00Sent


This query joins and tables on and filters out the sent messages in the last 30 days. After grouping the result by the user's name, it counts the number of SMS messages sent by each user. The result is then ordered in descending order to quickly identify the top users.

To practice a related customer analytics question on DataLemur's free interactive SQL code editor, try this recently asked Microsoft SQL interview question: Microsoft SQL Interview Question: Teams Super User

SQL Question 2: Calculate the Average Duration of Calls per User, per Day

At Twilio, one of the ways we connect businesses with their consumers is by providing VOIP services. Suppose you are given a table called that logs every call made using Twilio's services by each client. In this table:

  • call_id is the unique identifier for the call
  • user_id is the identifier for the user who made the call
  • call_date is the date the call was made
  • call_duration is the duration of the call in seconds

Your task is to write a PostgreSQL SQL query that generates a new table showing the average call duration for each user, for each day.

Example Input:
Example Output:


This query uses the window function to calculate average call duration for each user (), for each . The clause ensures averages are calculated separately for each user, and each day. The resulting table shows the average call duration in seconds for each user, for every day they made a call.

Pro Tip: Window functions are a popular SQL interview topic, so practice every window function problem on DataLemur

SQL Interview Questions on DataLemur

SQL Question 3: Can you explain the purpose of the constraint and give an example of when you might use it?

The CHECK constraint is used to enforce rules on the data in a specific column. If a row is inserted or updated with data that does not follow the CHECK constraint's rule, the operation will fail.

For example, say you had a database that stores ad campaign data from Twilio's Google Analytics account.

Here's what some constraints could look like:

In this example, the constraint is used to ensure that the "budget" and "cost_per_click" fields have positive values. This helps to ensure that the data in the database is valid and makes sense in the context of ad campaigns.

You can also use the constraint to ensure that data meets other specific conditions. For example, you could use a constraint to ensure that the "start_date" is before the "end_date" for each ad campaign.

Twilio SQL Interview Questions

SQL Question 4: Call Traffic Analysis

As a Data Analyst at Twilio, an API platform for communication, one of your responsibilities could be to analyze the volume of telephony traffic (voice calls and SMS messages) in order to spot trends, anomalies and optimise resource allocation. Your task is to design a database schema to track the traffic data. Assume that each call or message is represented by a unique event with the following attributes: event_id, user_id, type (voice call or SMS), destination number, start_time, end_time (only for voice calls), and content (only for SMS).

Your challenge is to determine which user reached the highest total duration of calls within the last month and to calculate how many SMS they have sent during the same period.

Example Input:
101111voice987654321002/01/2022 15:12:1002/01/2022 15:15:30NULL
102222SMS987654321102/02/2022 11:10:00NULLHello, how are you?
103111voice987654321202/03/2022 13:15:0002/03/2022 14:15:00NULL
104111SMS987654321002/04/2022 10:12:00NULLWhere are you?
105222voice987654321302/05/2022 20:15:0002/05/2022 20:30:00NULL


This query firstly selects all voice call events from the last month and sums up the duration for each user. Then, it finds the most active user (in terms of total call duration). Finally, it counts the number of SMS events for the same user and time period.

SQL Question 5: What does the SQL function do?

The function can take in multiple paramaters, and returns the first input paramater that is not null. If all arguments are null, the COALESCE function will return null too.

Suppose you have a table of Twilio salespeople and the number of deals they closed. If a salesperson didn't close any deals, the data from the 3rd-party CRM system exports a NULL value.

Jason WrightNULL
Drew Jackson3
Chris HoNULL
Adam Cohen2
Samantha Perez4

To change these NULLs to zeros, you can use the function in the following way:

This would result in the following data:

Jason Wright0
Drew Jackson3
Chris Ho0
Adam Cohen2
Samantha Perez4

SQL Question 6: Filtering Twilio's Customers

Given the table, write a query that returns all customers in the 'USA' who activated their account after '01/01/2021' and have usage_type set as 'international'. When structuring the data list all customers in ascending order by activation_date and then alphabetically by name.

Example Input:
Example Output:


This SQL query uses a WHERE clause to set three conditions. First, it filters for customers from 'USA'. Second, it filters out customers who activated their account after '01/01/2021'. Finally, it filters for customers with international usage type. The ORDER BY clause then sorts the remaining customers in ascending order by their activation_date and then alphabetically by name.

SQL Question 7: What's the difference between a unique index and non-unique index?

Some similarities between unique and non-unique indexes include:

  1. Both types improve the performance of SQL queries by providing a faster way to lookup the desired data.
  2. Both types use an additional data structure to store the indexed data, which requires additional storage space which impacts write performance.
  3. Both types of indexes can be created on one or more columns of a table.

Some differences between unique and non-unique indexes include:

  1. A unique index enforces the uniqueness of the indexed columns, meaning that no duplicate values are allowed in the indexed columns. A non-unique index allows duplicate values in the indexed columns.
  2. A unique index can be used to enforce the primary key of a table, but a non-unique index cannot.
  3. A unique index can have a maximum of one NULL value in the indexed columns, but a non-unique index can have multiple NULLs

SQL Question 8: Calculate the Total Usage of Different Twilio Services

Twilio is a cloud communications platform. It provides software developers with APIs to make and receive phone calls, send and receive text messages, and perform other communication tasks.

Suppose you have a transactions table that contains information about every API call made to Twilio, including the service used (Call API, Message API, etc.), the date of the call, and the duration of the call in seconds.

Can you write a SQL query to provide a summary of the total usage in hours for each service, for each month?

Example Input:

1205Call API2022-06-083600
1267Message API2022-07-1015
1350Call API2022-07-181800
1472Message API2022-07-2645
1583Chat API2022-06-0560

Example Output:

Call API61
Message API70.0166667
Call API70.5
Chat API60.0166667


This SQL query first extracts the month from the date of the transaction using the EXTRACT function. Then, the total duration in seconds for each service for each month is summed up and divided by 3600 (which is the number of seconds in an hour), giving the total duration in hours. Finally, this query uses a GROUP BY clause to provide the result for each service for each month.

SQL Question 9: Customer Interaction Analysis

Twilio, a cloud communications platform, has two tables. One table 'customers' stores information about their customers, including their id, name, and the creation timestamp of their accounts. Another table 'messages' stores data about the messages sent by these customers, including the unique message id, the id of the customer who sent it, the content of the message, and the time it was sent.

Customers want a way to discern active users from dormant ones. As a result, they've asked you to write a SQL query that joins these two tables and finds out which customers haven't sent a message in the last month as of a given date.

Use the following sample data to craft your query:

Example Input:
1Alice2020-01-01 00:00:00
2Bob2020-05-10 00:00:00
3Carol2020-03-12 00:00:00
4Dave2020-06-18 00:00:00
Example Input:
1011Hello, Twilio!2022-07-01 00:00:00
1022I love your services.2022-07-15 00:00:00
1033Great platform.2022-05-10 00:00:00
1044I recommend Twilio.2022-07-18 00:00:00
1052Keep up the good work.2022-08-13 00:00:00


The following PostgreSQL query should provide the necessary data:

This query considers both 'messages' and 'customers' tables. It will return all customers who haven't sent a message within the last month, up until today's date (represented by ). If a customer has never sent a message, their timestamp is used to determine if it has been more than a month since they joined Twilio. If so, they're considered "dormant" and included in the result set.

Because joins come up so often during SQL interviews, try an interactive Spotify JOIN SQL question: SQL join question from Spotify

SQL Question 10: What does it mean to perform a self-join?

A self-join is a operation in which a single table is joined to itself. To perform a self-join, you must specify the table name twice in the clause, giving each instance a different alias. You can then join the two instances of the table using a clause, and specify the relationship between the rows in a clause.

Think of using a self-joins whenever your data analysis involves analyzie pairs of the same things, like comparing the salaries of employees within the same department, or identifying pairs of products that are frequently purchased together (which you can do in this real SQL question from a Walmart interview).

For another self-join example, suppose you were conducting an HR analytics project and needed to examine how frequently employees within the same department at Twilio interact with one another, you could use a self-join query like the following to retrieve all pairs of Twilio employees who work in the same department:

This query returns all pairs of Twilio employees who work in the same department, and excludes pairs where the employee's id is the same (since this would represent the same Twilio employee being paired with themselves).

Preparing For The Twilio SQL Interview

The best way to prepare for a SQL interview, besides making sure you have strong SQL fundamentals, is to practice a ton of real SQL questions that were asked in recent job interviews. Besides solving the earlier Twilio SQL interview questions, you should also solve the 200+ SQL coding questions which come from companies like FAANG and tech startups. DataLemur Question Bank

Each SQL question has hints to guide you, step-by-step solutions and most importantly, there's an online SQL code editor so you can right online code up your SQL query answer and have it checked.

To prep for the Twilio SQL interview it is also wise to solve SQL questions from other tech companies like:

But if your SQL query skills are weak, forget about diving straight into solving questions – refresh your SQL knowledge with this free SQL for Data Analytics course.

SQL interview tutorial

This tutorial covers SQL concepts such as UNION and RANK() window functions – both of which come up routinely during Twilio SQL assessments.

Twilio Data Science Interview Tips

What Do Twilio Data Science Interviews Cover?

In addition to SQL interview questions, the other topics to prepare for the Twilio Data Science Interview are:

Twilio Data Scientist

How To Prepare for Twilio Data Science Interviews?

The best way to prepare for Twilio Data Science interviews is by reading Ace the Data Science Interview. The book's got:

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