8 Charles River SQL Interview Questions (Updated 2024)

Updated on

October 31, 2024

Charles River employees use SQL to extract and analyze experiment data, helping them evaluate the effectiveness of new drug treatments and make research more efficient. They also manage databases for various research projects, ensuring data is accurate and easily accessible, this is the reason why Charles River includes SQL questions in interviews for Data Science, Data Engineering, and Data Analytics roles.

So, to help you study, we've curated 8 Charles River Laboratories International SQL interview questions – able to solve them?

Charles River SQL Interview Questions

8 Charles River Laboratories International SQL Interview Questions

SQL Question 1: Average Trade Volume by Security

Assume you are a data analyst at Charles River, a leading financial technology firm. You are provided with a dataset representing trades made by various clients. Each record represents a distinct trade featuring attributes such as , , , and . Your task is to write a SQL query to calculate the rolling 3-day average trade volume per security.

Example Input:

trade_idclient_idtrade_datesecurity_idvolume
8502252022-09-01345253200
8301322022-09-0223144550
8123122022-09-02345253100
7814452022-09-0342352675
7205122022-09-03345253150
6300762022-09-04231445200
5981232022-09-04423526100

Example Output:

datesecurityavg_volume
2022-09-03345253150
2022-09-04231445125
2022-09-0442352687.5

Answer:


This query calculates the rolling 3-day (inclusive of current day) average trade volume for each security. It uses the window function with to segment data by and to sort data by from oldest to latest. specifies the range of rows used in the calculation of the average for each row - the current row and the two preceding rows for each security (3-day window).

p.s. Window functions show up super frequently during SQL interviews, so practice the 27+ window function questions on DataLemur

DataLemur SQL Questions

Explore the recent announcements from Charles River that highlight their commitment to advancing drug discovery and development! Keeping up with Charles River's news can help you appreciate their role in supporting the life sciences sector.

SQL Question 2: Department Salaries

Imagine there was a table of Charles River employee salaries, along with which department they were in. Write a query to compare the average salary of employees in each department to the company's average salary. Return the comparison result as 'higher', 'lower', or 'same' for each department. Display the department ID, payment month (in MM-YYYY format), and the comparison.

Write a SQL query for this question directly within the browser on DataLemur:

Department vs. Company Salary

The solution is LONG – 30+ lines of SQL. You can find a step-by-step solution here: Department Salaries.

SQL Question 3: How do you identify records in one table that are not present in a second table?

To discover records in one table that are not present in another, you can utilize a and filter out any values in the right-side table.

For example, say you had a table of Charles River customers and a 2nd table of all purchases made with Charles River. To find all customers who did not make a purchase, you'd use the following


This query returns all rows from the customers table, along with any matching rows from the purchases table. If there is no matching row in the purchases table, values will be returned for all of the right table's columns. The clause then filters out any rows where the purchases.id column is , leaving only customers who have not made a purchase.

Charles River Laboratories International SQL Interview Questions

SQL Question 4: Find the Average Transaction Amount per Customer

You are a data analyst at Charles River, a firm providing products and services for drug discovery tasks. There are two primary tables you have access to: and . The table has a and fields. The table has , , , and fields. Your objective is to calculate the average transaction amount per customer.

table Example Input:

customer_idname
1John Doe
2Jane Doe
3Mary Johnson

table Example Input:

transaction_idcustomer_idtransaction_dateamount
500112022-08-01$50
500222022-08-02$75
500322022-08-03$125
500432022-08-01$100
500532022-08-02$200
500612022-08-02$60
500712022-08-03$70

Answer:


This PostgreSQL query will join the and tables on the field and then group the data by customer. For each customer, the query calculates the average amount of their transactions using the function. The results are returned in a new set with customer details along with their average transaction amount.

The final output from this query will look like:

customer_idnameavg_transaction_amount
1John Doe$60
2Jane Doe$100
3Mary Johnson$150

To practice a very similar question try this interactive Uber User's Third Transaction Question which is similar for requiring analysis on users' transaction data or this Stripe Repeated Payments Question which is similar for involving transaction amounts.

SQL Question 5: What does it mean to use a constraint in a database?

A constraint ensures that all values in a column are different. It is often used in conjunction with other constraints, such as NOT NULL, to ensure that the data meets certain conditions.

For example, if you had Charles River employee data stored in a database, here's some constraints you'd use:


In the Charles River employee example, the constraint is applied to the "email" field to ensure that each employee has a unique email address. This helps to ensure the integrity of the data in the database and prevents errors that could occur if two employees had the same email address.

SQL Question 6: Click-Through-to-Purchase Conversion Rates for Charles River

Charles River, a digital product company, would like to analyze its click-through-to-purchase conversion rates. The company records every event when a user clicks an advertisement (), views a product page (), and adds a product to their cart (). Calculate the click-through-to-purchase conversion rate for each advertisement.

Provided are three sample tables in the markdown format: , , and .

Example Input:

click_iduser_idclick_datead_id
12106/08/2022 00:00:00101
23506/10/2022 00:00:00102
34506/18/2022 00:00:00103
42107/26/2022 00:00:00104
53307/05/2022 00:00:00105

Example Input:

view_iduser_idview_datead_idproduct_id
12106/08/2022 01:00:00101201
23506/10/2022 05:00:00102202
34506/18/2022 08:00:00103203
42107/26/2022 12:00:00104204
53307/05/2022 15:00:00105205

Example Input:

add_iduser_idadd_dateproduct_id
12106/08/2022 02:00:00201
23506/10/2022 00:00:00202
34506/20/2022 00:00:00203
42108/01/2022 00:00:00204
53307/10/2022 00:00:00205

Answer:


This PostgreSQL query calculates the click-through-to-purchase conversion rates for each advertisement by dividing the count of unique cart_add_ids by the count of unique ad_click_ids. The is used to connect all tables based on and the relevant identifiers ( and ). This ensures that all ad clicks are considered, even if they did not result in a product view or cart addition.

To practice another question about calculating rates, try this TikTok SQL Interview Question within DataLemur's online SQL coding environment:

TikTok SQL question

SQL Question 7: What does adding 'DISTINCT' to a SQL query do?

The clause is used to remove all duplicate records from a query.

For example, if you had a table of open jobs Charles River was hiring for, and wanted to see what are all the unique job titles that were currently available at the company, you could write the following query:


SQL Question 8: Retrieve Customer and Corresponding Purchase Information

Write a query that retrieves customer information alongside their purchases and outputs a list showing customer name, purchase date, purchase ID, and the product name they purchased. Join the customers table with the purchase table on the customer id, and the products table on the product id. Only return rows where the purchase date is after January 1, 2022.

Example Input:

customer_idfirst_namelast_name
101MariaDavis
102JamesJohnson
103LindaMiller
104RobertRodriguez
105PatriciaGarcia

Example Input:

purchase_idcustomer_idpurchase_dateproduct_id
20110102/10/2022301
20210201/15/2022302
20310303/23/2022303
20410402/28/2022304
20510512/20/2021305

Example Input:

product_idproduct_namecategory_id
301iPhone 121
302Samsung Galaxy S211
303MacBook Pro2
304Dell XPS 132
305iPad Pro3

Answer:


The above query first joins the table with on , and then joins the resulting table with on . It then selects the necessary columns and only includes records where the is after January 1, 2022.

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

Spotify JOIN SQL question

Charles River SQL Interview Tips

The best way to prepare for a Charles River SQL interview is to practice, practice, practice. Beyond just solving the earlier Charles River SQL interview questions, you should also solve the 200+ DataLemur interview questions which come from companies like Microsoft, Google, Amazon, and tech startups.

DataLemur SQL Interview Questions

Each problem on DataLemur has multiple hints, full answers and best of all, there is an online SQL code editor so you can instantly run your SQL query and have it checked.

To prep for the Charles River SQL interview you can also be useful to solve SQL questions from other healthcare and pharmaceutical companies like:

In case your SQL skills are weak, don't worry about going right into solving questions – refresh your SQL knowledge with this free SQL tutorial.

DataLemur SQL Course

This tutorial covers topics including handling timestamps and aggreage functions like MIN()/MAX() – both of which pop up often in Charles River interviews.

Charles River Laboratories International Data Science Interview Tips

What Do Charles River Data Science Interviews Cover?

Beyond writing SQL queries, the other topics covered in the Charles River Data Science Interview are:

Charles River Data Scientist

How To Prepare for Charles River Data Science Interviews?

I think the optimal way to study for Charles River Data Science interviews is to read the book Ace the Data Science Interview.

It has 201 interview questions taken from FAANG (FB, Apple, Amazon, Netflix, Google). The book's also got a refresher on Python, SQL & ML. And finally it's helped a TON of people, which is why it's earned 1000+ reviews on Amazon and a 4.5-star rating.

Ace the DS Interview

While the book is more technical in nature, it's also important to prepare for the Charles River behavioral interview. A good place to start is by reading the company's values and mission.

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