Prepare for and practice interview questions from Annapurna Labs.

Annapurna Labs Interview Questions

Annapurna Labs Interview Guides

Data Structures & Algorithms

Given two sorted lists, write a function to merge them into one sorted list.

Merge Sorted Lists

Find the missing integer from a array of consequtive integers

Find the Missing Number

Write a function n_frequent_words that returns the top N frequent words and their frequencies, and state its run-time

Get Top N Frequent Words

Given a list of integers, find the index at which the sum of the left half of the list is equal to the right half.

Equivalent Index

Given an integer N, write a function that returns all of the prime numbers up to N

Prime to N

Machine Learning

Which model would perform better and why?

Booking Regression

How do we deal with the missing square footage data to construct our model

Missing Housing Data

Why can the same machine learning algorithm produce different success rates on the same dataset

Same Algorithm Different Success

Lasso vs Ridge

In which case would you use a bagging algorithm versus a boosting algorithm

Bagging vs Boosting

Select the 2nd highest salary in the engineering department

2nd Highest Salary

Paired Products

Write a query to get the number of customers that were upsold

Upsell Transactions

Create a report displaying which shipments were delivered to customers during their membership period.

Completed Shipments

Daily Active Users

Statistics

Say you are given a dataset of perfectly linearly separable data. What would happen when you run logistic regression?

Perfectly Separable

How many more samples are needed to decrease the margin of error to 0.3

Reducing Error Margin

What are the assumptions of linear regression?

Assumptions of Linear Regression

What is the downside of only using the R-Squared (R^2) value to determine a relationship between two variables

Using R Squared

Explain what an unbiased estimator is and provide a layman-friendly example

Unbiased Estimator

Analytics

What metrics would you use to determine the value of each marketing channel?

Marketing Channel Metrics

We're interested in how user activity affects user purchasing behavior. 

Activity Conversion

Let’s say that you're in charge of an e-commerce D2C business that sells socks. What business health metrics would you care?

D2C Socks e-Commerce

How would you determine customer service quality through a chat box?

Measuring Customer Service Quality

Challenges of specific student test score layouts, recommended formatting changes for enhanced analysis, and common issues found in "messy" datasets.

Digitizing Student Test Scores

<p>Write a SQL query to select the 2nd highest salary in the engineering department.</p>

<p><strong>Note:</strong> If more than one person shares the highest salary, the query should select the next highest salary.</p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<p><code>employees</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>first_name</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>last_name</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>salary</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>department_id</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><code>departments</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>name</code></td>
<td>VARCHAR</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>salary</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>


<p>Given two sorted lists, write a function to merge them into one sorted list.</p>

<p><em>Bonus: What’s the time complexity?</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">list1</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span>
</span></span><span class="line"><span class="cl"><span class="n">list2</span> <span class="o">=</span> <span class="p">[</span><span class="mi">2</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">6</span><span class="p">]</span>
</span></span></code></pre>

<p><strong>Output:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="k">def</span> <span class="nf">merge_list</span><span class="p">(</span><span class="n">list1</span><span class="p">,</span><span class="n">list2</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">6</span><span class="p">]</span>
</span></span></code></pre>


<p>You have two sorted lists, one with m elements and one with n elements. What would be the time complexity of merging the two lists via merge sort?</p>


You have two sorted lists, one with m elements and one with n elements. What would be the time complexity of merging the two lists via merge sort? 

Merging Lists Complexity

<p>We’re given a table of product purchases. Each row in the table represents an individual user product purchase.</p>

<p>Write a query to get the number of customers that were upsold, or in other words, the number of users who bought additional products after their first purchase.</p>

<p><em>Note: If the customer purchased two things on the same day, that does not count as an upsell, as they were purchased within a similar timeframe.</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<p><code>transactions</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>created_at</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>product_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>quantity</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>num_of_upsold_customers</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>


<p>Given an integer <code>N</code>, write a function that returns a list of all of the prime numbers up to <code>N</code>.</p>

<p><em>Note: Return an empty list there are no prime numbers less than or equal to <code>N</code>.</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">N</span> <span class="o">=</span> <span class="mi">3</span>
</span></span></code></pre>

<p><strong>Output:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="k">def</span> <span class="nf">prime_numbers</span><span class="p">(</span><span class="n">N</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="p">[</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]</span>
</span></span></code></pre>


<p>Let’s say we have two tables, <code>transactions</code> and <code>products</code>. Hypothetically the <code>transactions</code> table consists of over a billion rows of purchases bought by users.</p>

<p>We are trying to find paired products that are often purchased together by the same user, such as wine and bottle openers, chips and beer, etc..</p>

<p>Write a query to find the top five paired products and their names.</p>

<p><em>Notes:</em>
   <em>For the purposes of satisfying the test case, p2 should be the item that comes first in the alphabet.</em>
   <em>The <code>qty</code> column represents paired products count</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<p><code>transactions</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>created_at</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>product_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>quantity</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><code>products</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>name</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>price</code></td>
<td>FLOAT</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>p1</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>p2</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>qty</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>


<p>You have an array of integers, <code>nums</code> of length <code>n</code> spanning <code>0</code> to <code>n</code> with one missing. Write a function <code>missing_number</code> that returns the missing number in the array.</p>

<p><em>Note: Complexity of $O(n)$ required.</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">nums</span> <span class="o">=</span> <span class="p">[</span><span class="mi">0</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span> 
</span></span><span class="line"><span class="cl"><span class="n">missing_number</span><span class="p">(</span><span class="n">nums</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="mi">3</span>
</span></span></code></pre>


<p>You’re given a string that may contain the characters <strong><code>{</code></strong>, <strong><code>}</code></strong>, <strong><code>[</code></strong>, <strong><code>]</code></strong>, <strong><code>(</code></strong>, and <strong><code>)</code></strong>.</p>

<p><strong>Task:</strong> Verify that the string is balanced. A balanced string is one where every opening character, <strong><code>{</code></strong>, <strong><code>[</code></strong>, or <strong><code>(</code></strong>, has a corresponding closing character, <strong><code>}</code></strong>, <strong><code>]</code></strong>, or <strong><code>)</code></strong>.</p>

<p>Write a function called <strong><code>is_balanced(string: str) -&gt; bool</code></strong> which verifies the balance of a string.</p>

<p><strong>Example:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">is_balanced</span><span class="p">(</span><span class="s1">&#39;(())[]</span><span class="si">{}</span><span class="s1">&#39;</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="kc">True</span>
</span></span></code></pre>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">is_balanced</span><span class="p">(</span><span class="s1">&#39;{([()</span><span class="si">{}</span><span class="s1">])()}&#39;</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="kc">True</span>
</span></span></code></pre>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">is_balanced</span><span class="p">(</span><span class="s1">&#39;</span><span class="si">{}</span><span class="s1">[]())&#39;</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="kc">False</span>
</span></span></code></pre>

<hr/>


Write a function that tests whether a string of brackets is balanced.

The Brackets Problem

<p>Let’s say we have a table with an <code>id</code> and <code>name</code> fields. The table holds over 100 million rows and we want to sample a random row in the table without throttling the database.</p>

<p>Write a query to randomly sample a row from this table.</p>

<p><strong>Input:</strong></p>

<p><code>big_table</code> table</p>

<table>
<thead>
<tr>
<th>Columns</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>name</code></td>
<td>VARCHAR</td>
</tr>
</tbody>
</table>


Write a query to randomly sample a row from a big table.

Random SQL Sample

<p>You have a table containing information about the time that employees have worked on a project.</p>

<p>Write a query to find how many unique calendar days each employee worked. Order your query by the <code>employee_id</code>.</p>

<p><strong>Example:</strong></p>

<p>An employee worked on two projects, and recorded their time like this:</p>

<table>
<thead>
<tr>
<th>employee_id</th>
<th>project_id</th>
<th>start_date</th>
<th>end_date</th>
</tr>
</thead>

<tbody>
<tr>
<td>1	</td>
<td>1</td>
<td>2024-01-01</td>
<td>2024-01-06</td>
</tr>

<tr>
<td>1	</td>
<td>2</td>
<td>2024-01-03</td>
<td>2024-01-07</td>
</tr>
</tbody>
</table>
<p>The number of unique calendar days is therefore 7.</p>

<h3>Schema</h3>

<p><strong>Input:</strong></p>

<p><code>projects</code> table</p>

<table>
<thead>
<tr>
<th>Columns	</th>
<th>	Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>employee_id</code>	</td>
<td>	INTEGER	</td>
</tr>

<tr>
<td><code>project_id</code>	</td>
<td>	INTEGER	</td>
</tr>

<tr>
<td><code>start_date</code>	</td>
<td>	DATE	</td>
</tr>

<tr>
<td><code>end_date</code>	</td>
<td>	DATE	</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Columns	</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>employee_id</code>	</td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>unique_days_worked</code>	</td>
<td>INTEGER</td>
</tr>
</tbody>
</table>

<h3>Example</h3>

<p><strong>Input:</strong></p>

<p><strong><code>projects</code> table</strong></p>

<table>
<thead>
<tr>
<th>employee_id</th>
<th>project_id</th>
<th>start_date</th>
<th>end_date</th>
</tr>
</thead>

<tbody>
<tr>
<td>1</td>
<td>1</td>
<td>2024-01-01</td>
<td>2024-01-15</td>
</tr>

<tr>
<td>1</td>
<td>1</td>
<td>2024-01-21</td>
<td>2024-01-31</td>
</tr>

<tr>
<td>1</td>
<td>2</td>
<td>2024-01-25</td>
<td>2024-02-28</td>
</tr>

<tr>
<td>2</td>
<td>3</td>
<td>2024-01-01</td>
<td>2024-01-31</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>employee_id</th>
<th>unique_days_worked</th>
</tr>
</thead>

<tbody>
<tr>
<td>1</td>
<td>54</td>
</tr>

<tr>
<td>2</td>
<td>31</td>
</tr>
</tbody>
</table>


Unique Work Days

<p>Tell me about a data project that didn’t go the way you expected. What did you set out to do, what surprised you, and how did you handle it?</p>


Describing a data project and its challenges

Hurdles In Data Projects

Behavioral

<p>Given a <strong><code>ListNode</code></strong> representing the head of a linked list, write a function <strong><code>is_cyclic</code></strong> which returns <strong><code>True</code></strong> if a linked list is cyclic and <strong><code>False</code></strong> if it is not. A linked list is cyclic when there is no tail to the linked list, and the supposed tail is attached to another node inside the list itself, creating a cycle.</p>

<p>A <strong><code>ListNode</code></strong> is defined as:</p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="k">class</span> <span class="nc">ListNode</span><span class="p">:</span>
</span></span><span class="line"><span class="cl">	<span class="n">value</span><span class="p">:</span> <span class="n">ListNode</span> <span class="o">=</span> <span class="kc">None</span>
</span></span><span class="line"><span class="cl">  <span class="nb">next</span><span class="p">:</span> <span class="n">ListNode</span> <span class="o">=</span> <span class="kc">None</span>
</span></span></code></pre>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<p><img src="/s3/questions/1a65651d-a483-4481-b073-2738d65ab9d7.png" alt="image"/></p>

<p><strong>Output:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="k">def</span> <span class="nf">is_cyclic</span><span class="p">(</span><span class="n">head</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="kc">True</span>
</span></span></code></pre>


Cyclic Detection

<p>Say you are tasked with analyzing how well a model fits the data given. You want to determine a relationship between two variables.</p>

<p>What is the downside of only using the R-Squared $(R^2)$ value to do so?</p>


<p>In the context of regression analysis, why is it misleading to rely solely on a high R-squared ($R^2$) value to infer a causal relationship between variables?</p>

In the context of regression analysis, why is it misleading to rely solely on a high R-squared ($R^2$) value to infer a causal relationship between variables?

Using R Squared I

<p>Which of the following best describes the R-squared ($R^2$) value in the context of a regression model?</p>

Which of the following best describes the R-squared ($R^2$) value in the context of a regression model?

Using R Squared II

<p>What is a limitation of using R-squared ($R^2$) as the only metric to evaluate a regression model's fit?</p>

What is a limitation of using R-squared ($R^2$) as the only metric to evaluate a regression model's fit?

Using R Squared III

<p>Given a table of account statuses, write a query to get the percentage of accounts that were active on December 31st, 2019, and closed on January 1st, 2020, over the total number of accounts that were active on December 31st.
Each account has only one daily record indicating its status at the end of the day.</p>

<p><em>Note: Round the result to 2 decimal places.</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<p><code>account_status</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>account_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>date</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>status</code></td>
<td>VARCHAR</td>
</tr>
</tbody>
</table>

<table>
<thead>
<tr>
<th>account_id</th>
<th>date</th>
<th>status</th>
</tr>
</thead>

<tbody>
<tr>
<td>1</td>
<td>2020-01-01</td>
<td>closed</td>
</tr>

<tr>
<td>1</td>
<td>2019-12-31</td>
<td>open</td>
</tr>

<tr>
<td>2</td>
<td>2020-01-01</td>
<td>closed</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>percentage_closed</code></td>
<td>FLOAT</td>
</tr>
</tbody>
</table>


Write a query to get the percentage of accounts active on December 31st, 2019 and closed on January 1st, 2020 over accounts active on December 31st, rounded to 2 decimals

Closed Accounts

<p>To finish a class, students must pass four exams (exam ids: 1,2,3 and 4).</p>

<p>Given a table <code>exam_scores</code> containing the data about all of the exams that students took, form a new table to track the scores for each student.</p>

<p><em>Note: Students took each exam only once.</em></p>

<p><strong>Example:</strong></p>

<p>For the given input:</p>

<table>
<thead>
<tr>
<th><code>student_id</code></th>
<th><code>student_name</code></th>
<th><code>exam_id</code></th>
<th><code>score</code></th>
</tr>
</thead>

<tbody>
<tr>
<td>100</td>
<td><code>Anna</code></td>
<td>1</td>
<td>71</td>
</tr>

<tr>
<td>100</td>
<td><code>Anna</code></td>
<td>2</td>
<td>72</td>
</tr>

<tr>
<td>100</td>
<td><code>Anna</code></td>
<td>3</td>
<td>73</td>
</tr>

<tr>
<td>100</td>
<td><code>Anna</code></td>
<td>4</td>
<td>74</td>
</tr>

<tr>
<td>101</td>
<td><code>Brian</code></td>
<td>1</td>
<td>65</td>
</tr>
</tbody>
</table>
<p>the expected output should be:</p>

<table>
<thead>
<tr>
<th><code>student_name</code></th>
<th><code>exam_1</code></th>
<th><code>exam_2</code></th>
<th><code>exam_3</code></th>
<th><code>exam_4</code></th>
</tr>
</thead>

<tbody>
<tr>
<td><code>Anna</code></td>
<td>71</td>
<td>72</td>
<td>73</td>
<td>74</td>
</tr>

<tr>
<td><code>Brian</code></td>
<td>65</td>
<td>NULL</td>
<td>NULL</td>
<td>NULL</td>
</tr>
</tbody>
</table>
<p><strong>Input:</strong></p>

<p><code>exam_scores</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>student_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>student_name</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>exam_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>score</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>student_name</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>exam_1</code></td>
<td>INT</td>
</tr>

<tr>
<td><code>exam_2</code></td>
<td>INT</td>
</tr>

<tr>
<td><code>exam_3</code></td>
<td>INT</td>
</tr>

<tr>
<td><code>exam_4</code></td>
<td>INT</td>
</tr>
</tbody>
</table>


List out the exams sources of each student in MySQL

Exam Scores

<p>How does random forest generate the forest? Additionally, why would we use it over other algorithms such as logistic regression?</p>


<p>What happens when you average the output on multiple decision trees?</p>


What happens when you average the output on multiple decision trees?

Average Trees

How does random forest generate the forest and why use it over logistic regression

Random Forest Explanation

<p>We’re given two tables. One is named <code>projects</code> and the other maps employees to the projects they’re working on.</p>

<p>Write a query to get the top five most expensive projects by budget to employee count ratio.</p>

<p><em>Note: Exclude projects with 0 employees. Assume each employee works on only one project.</em></p>

<p><code>projects</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>title</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>state_date</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>end_date</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>budget</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><code>employee_projects</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>project_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>employee_id</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>title</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>budget_per_employee</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>


Write a query to get the top five projects by budget-to-employee-count ratio

Employee Project Budgets

<p>Say you are tasked with designing a data mart or data warehouse for a new online retailer.</p>

<p>How would you design the system?</p>

<p><em>Note: Sketch a star schema to explain your design.</em></p>


Design a data warehouse for a new online retailer

Retailer Data Warehouse

Data Modeling

<p>As an accountant for a local grocery store, you have been tasked with calculating the daily sales of each product since their last restocking.</p>

<p>You have been provided with three tables: <code>products</code>, <code>sales</code>, and <code>restocking</code>. The <code>products</code> table contains information about each product, the <code>sales</code> table records the sales transactions, and the <code>restocking</code> table tracks the restocking events.</p>

<p>Write a query to retrieve the running total of sales for each product since its last restocking event.</p>

<p><strong><em>Notes:</em></strong> <em>The <code>sales_since_last_restock</code> column represents the cumulative sales of the product for each day since its last restocking event. Order the result set by <code>product_id</code>.</em></p>

<p>Example:</p>

<p><strong>Input:</strong></p>

<p>Table <code>products</code>:</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td>product_id</td>
<td>int</td>
</tr>

<tr>
<td>product_name</td>
<td>varchar</td>
</tr>
</tbody>
</table>
<p>Table <code>sales</code>:</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td>sales_id</td>
<td>int</td>
</tr>

<tr>
<td>product_id</td>
<td>int</td>
</tr>

<tr>
<td>date</td>
<td>date</td>
</tr>

<tr>
<td>sold_count</td>
<td>int</td>
</tr>
</tbody>
</table>
<p>Table <code>restocking</code>:</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td>restock_id</td>
<td>int</td>
</tr>

<tr>
<td>product_id</td>
<td>int</td>
</tr>

<tr>
<td>restock_date</td>
<td>date</td>
</tr>
</tbody>
</table>
<p><strong>Output</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td>product_name</td>
<td>varchar</td>
</tr>

<tr>
<td>date</td>
<td>date</td>
</tr>

<tr>
<td>sales_since_last_restock</td>
<td>int</td>
</tr>
</tbody>
</table>


Calculate daily sales of each product since last restocking.

Cumulative Sales Since Last Restocking

<p>Given a list of integers, find the index at which the sum of the left half of the list is equal to the right half.</p>

<p>If there is no index where this condition is satisfied return -1.</p>

<p><strong>Example 1:</strong></p>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">nums</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">]</span>
</span></span></code></pre>

<p><strong>Output:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">equivalent_index</span><span class="p">(</span><span class="n">nums</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="mi">2</span>
</span></span></code></pre>

<p><strong>Example 2:</strong></p>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">nums</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span>
</span></span></code></pre>

<p><strong>Output:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">equivalent_index</span><span class="p">(</span><span class="n">nums</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="o">-</span><span class="mi">1</span>
</span></span></code></pre>


<p>Let’s say that you’re a data scientist working on a distribution team at Amazon.</p>

<p>Create a report displaying which shipments were delivered to customers during their membership period.</p>

<p>If the transaction is shipped during the membership period, column <code>is_member</code> should have the value <code>Y</code>. If not, the column should have the value <code>N</code>.</p>

<p><em>Hint: You may assume that <code>customer_id</code> is unique and that shipments are made only to registered customers.</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<p><code>customers</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>customer_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>membership_start_date</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>membership_end_date</code></td>
<td>DATETIME</td>
</tr>
</tbody>
</table>
<p><code>shipments</code> table</p>

<table>
<thead>
<tr>
<th>Column</th>
<th>type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>shipment_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>ship_date</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>customer_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>quantity</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>shipment_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>ship_date</code></td>
<td>DATETIME</td>
</tr>

<tr>
<td><code>customer_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>is_member</code></td>
<td>TEXT</td>
</tr>

<tr>
<td><code>quantity</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>


<p>Given the integer list nums with length n, create a function that converts each integer in the list into its corresponding Roman numeral representation. The Roman numeral symbols and corresponding values are provided in the table below. The function must be able to handle integers up to 1000.</p>

<table>
<thead>
<tr>
<th>Symbols</th>
<th>Values</th>
</tr>
</thead>

<tbody>
<tr>
<td>I</td>
<td>1</td>
</tr>

<tr>
<td>IV</td>
<td>4</td>
</tr>

<tr>
<td>V</td>
<td>5</td>
</tr>

<tr>
<td>IX</td>
<td>9</td>
</tr>

<tr>
<td>X</td>
<td>10</td>
</tr>

<tr>
<td>XL</td>
<td>40</td>
</tr>

<tr>
<td>L</td>
<td>50</td>
</tr>

<tr>
<td>XC</td>
<td>90</td>
</tr>

<tr>
<td>C</td>
<td>100</td>
</tr>

<tr>
<td>CD</td>
<td>400</td>
</tr>

<tr>
<td>D</td>
<td>500</td>
</tr>

<tr>
<td>CM</td>
<td>900</td>
</tr>

<tr>
<td>M</td>
<td>1000</td>
</tr>
</tbody>
</table>

<h3><strong>Examples:</strong></h3>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">nums</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">179</span><span class="p">]</span>
</span></span></code></pre>

<p><strong>Output:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="p">[</span><span class="s2">&#34;I&#34;</span><span class="p">,</span> <span class="s2">&#34;IV&#34;</span><span class="p">,</span> <span class="s2">&#34;CLXXIX&#34;</span><span class="p">]</span>
</span></span></code></pre>


Convert a list of integers into their Roman numeral representations

Integer to Roman

<p>Let’s say we’re comparing two machine learning algorithms. In which case would you use a bagging algorithm versus a boosting algorithm? </p>

<p>Give an example of the tradeoffs between the two.</p>


<p>In machine learning, when would you use a bagging algorithm over a boosting algorithm?</p>


In machine learning, when would you use a bagging algorithm over a boosting algorithm?

Bagging vs. Boosting

<p>Let’s say we want to build a new delivery time estimate model for consumers ordering food delivery.</p>

<p>How would you determine if the new model predicts delivery times better than the old model?</p>


<p>When rolling out a new delivery time estimate model in production, what is an important consideration?</p>

When rolling out a new delivery time estimate model in production, what is an important consideration?

Delivery Estimate Model I

<p>What is one way to measure the performance of a new delivery time estimate model for food delivery?</p>

What is one way to measure the performance of a new delivery time estimate model for food delivery?

Delivery Estimate Model II

How would you determine if the new model predicts delivery times better than the old model

Delivery Estimate Model

A/B Testing

<p>Let’s say your manager is running A/B tests on a website. You find out that your manager has run an A/B test with 20 different variants and come back to you that one of the variants is significant.</p>

<p>Would you think there was anything fishy about the results?</p>


<p>Your manager reconfigured an A/B test from 2 to 20 different variants, while keeping all other parameters unchanged. They come back to you that one of the variants is significant.</p>

<p>Is this conclusion likely to be correct? Why or why not?</p>


Your manager reconfigured an A/B test from 2 to 20 different variants, while keeping all other parameters unchanged. They come back to you that one of the variants is significant.

Is this conclusion likely to be correct? Why or why not?

Twenty Variant Test

Would you think there was anything fishy about an A/B test run with 20 different variants where one is significant

Twenty Variants

<p>Let’s say we want to build a model to predict booking prices on Airbnb.</p>

<p>Between linear regression and random forest regression, which model would perform better and why?</p>


<p>What are the two key concepts of random forest regression?</p>

What are the two key concepts of random forest regression?

<p>There are two lists, <code>list X</code> and <code>list Y</code>. Both lists contain integers from <code>-1000</code> to <code>1000</code> and are identical to each other except that one integer is removed in <code>list Y</code> that exists in <code>list X</code>.</p>

<p>Write a function <code>one_element_removed</code> that takes in both lists and returns the integer that was removed in $O(1)$ space and $O(n)$ time without using the python set function.</p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="n">list_x</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span>
</span></span><span class="line"><span class="cl"><span class="n">list_y</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="n">one_element_removed</span><span class="p">(</span><span class="n">list_x</span><span class="p">,</span> <span class="n">list_y</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="mi">3</span>
</span></span></code></pre>


<p>Which of the following methods would be the most efficient way of identifying what element is not included in two lists of integers that differ by a single element being removed?</p>

<p>For example, if the lists are <code>[1,2,3,4,5]</code> and <code>[1,2,4,5]</code>, the function should return <code>3</code></p>


Which of the following methods would be the most efficient way of identifying what element is not included in two lists of integers that differ by a single element being removed? 

For example, if the lists are `[1,2,3,4,5]` and `[1,2,4,5]`, the function should return `3`

Efficiently Identifying Missing Integer

Write a function one_element_removed that returns the integer removed from list Y in O(1) space and O(n) time without using set

One Element Removed

<p>Let’s say you work at Allstate. Allstate is running <code>N</code>  online ads right now. The table <code>ads</code> contains all those ads, ranked by popularity via the <code>id</code> column (e.g., the entry with <code>id = 1</code> is the most popular, etc.).</p>

<p>Create a subquery or common table expression named <code>top_ads</code> containing the top 3 ads (by popularity) and return the number of rows that would result from the following operations</p>

<ol>
<li><code>ads INNER JOIN top_ads</code></li>
<li><code>ads LEFT JOIN top_ads</code></li>
<li><code>ads RIGHT JOIN top_ads</code></li>
<li><code>ads CROSS JOIN top_ads</code></li>
</ol>

<p><em>Note: Please make the <code>join_type</code> column in your output have the values <code>inner_join</code>, <code>left_join</code>, etc. for each of their respective join types</em></p>

<p><em>Note: Please return only one query with each number in a different row</em></p>

<p><strong>Example:</strong></p>

<p><strong>Input:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>name</code></td>
<td>VARCHAR</td>
</tr>
</tbody>
</table>
<p><strong>Output:</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td><code>join_type</code></td>
<td>VARCHAR</td>
</tr>

<tr>
<td><code>number_of_rows</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>


Create top_ads with the top 3 ads and return the row counts for inner, left, right, and cross joins with ads

Size of Joins

<p>Let’s say we have a sample size of $n$. The margin of error for our sample size is 3.</p>

<p>How many more samples would we need to decrease the margin of error to 0.3?</p>


<p>Let's say we have a sample size of N. The margin of error for our sample size is 3.</p>

How many more samples would we need to decrease the margin of error to 0.3?

Let's say we have a sample size of N. The margin of error for our sample size is 3.

How many more samples would we need to decrease the margin of error to 0.3?

<p>A digital health startup is analyzing ECG sensor data captured from wearable heart monitors. Each row in the dataset represents one heartbeat, including its timestamp and calculated heart rate at that moment.</p>

<p>The clinical team wants to detect tachycardia runs, periods where a patient’s heart rate stays above 100 bpm continuously for at least 10 seconds .</p>

<p>Write a SQL query to identify every valid tachycardia run and return:</p>

<ol>
<li><code>patient_id</code></li>
<li><code>run_start</code>: timestamp of the first beat in the run</li>
<li><code>run_end</code>: timestamp of the last beat in the run</li>
<li><code>duration_seconds</code>: difference in seconds between start and end (rounded to 2 decimals)</li>
</ol>

<p>Sort results by <code>patient_id</code>, then <code>run_start</code>.</p>

<p><strong>Definition of a Tachycardia Run</strong></p>

<ul>
<li>All consecutive heartbeats must have <code>heart_rate &gt; 100</code></li>
<li>Total duration ≥ 10 seconds</li>
<li>Any gap ≥ 3 seconds between consecutive beats breaks the run</li>
</ul>

<p><strong><em>Note:</em></strong> <em>A patient may have multiple separate runs.</em></p>

<p><strong>Schema</strong></p>

<p><strong>Input</strong></p>

<p><strong><code>ecg_readings</code> table</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td>patient_id</td>
<td>INTEGER</td>
</tr>

<tr>
<td>timestamp</td>
<td>DATETIME</td>
</tr>

<tr>
<td>heart_rate</td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><strong>Output</strong></p>

<table>
<thead>
<tr>
<th>Column</th>
<th>Type</th>
</tr>
</thead>

<tbody>
<tr>
<td>patient_id</td>
<td>INTEGER</td>
</tr>

<tr>
<td>run_start</td>
<td>DATETIME</td>
</tr>

<tr>
<td>run_end</td>
<td>DATETIME</td>
</tr>

<tr>
<td>duration_seconds</td>
<td>DECIMAL</td>
</tr>
</tbody>
</table>
<p><strong>Example</strong></p>

<p><strong>Input</strong></p>

<p><strong><code>ecg_readings</code> table</strong></p>

<table>
<thead>
<tr>
<th>patient_id</th>
<th>timestamp</th>
<th>heart_rate</th>
</tr>
</thead>

<tbody>
<tr>
<td>1</td>
<td>2025-03-01 10:00:00</td>
<td>98</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:02</td>
<td>105</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:04</td>
<td>110</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:07</td>
<td>108</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:12</td>
<td>107</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:15</td>
<td>85</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:20</td>
<td>120</td>
</tr>

<tr>
<td>1</td>
<td>2025-03-01 10:00:28</td>
<td>122</td>
</tr>
</tbody>
</table>
<p><strong>Output</strong></p>

<table>
<thead>
<tr>
<th>patient_id</th>
<th>run_start</th>
<th>run_end</th>
<th>duration_seconds</th>
</tr>
</thead>

<tbody>
<tr>
<td>1</td>
<td>2025-03-01 10:00:02</td>
<td>2025-03-01 10:00:12</td>
<td>10.00</td>
</tr>
</tbody>
</table>

<h3><strong>Explanation</strong></h3>

<p>The high-rate streak from 10:00:02 to 10:00:12 forms one continuous sequence because each gap between timestamps (2 to 4, 4 to 7, and 7 to 12) is less than three seconds. This creates a valid 10-second run. The later high values at seconds 20 and 28 are separated by an eight-second gap, so they form a different sequence that is shorter than 10 seconds and is therefore ignored.</p>


Identify tachycardia runs where heart rate stays above 100 bpm for at least 10 seconds and return run start, end, and duration

Detecting ECG Tachycardia Runs

<p>Say you are given a dataset of perfectly linearly separable data.</p>

<p>What would happen when you run logistic regression?</p>


<p>What happens when logistic regression is run on a dataset of perfectly linearly separable data?</p>

What happens when logistic regression is run on a dataset of perfectly linearly separable data?

Challenge

<p>Staff Software Engineer</p><p><strong>Location:</strong> Berlin (Hybrid)</p><p><strong>Salary:</strong> Up to €110,000 + Benefits</p><p><br></p><p>We’re partnering with one of Europe’s fastest-growing entertainment technology platforms, operating at huge scale across streaming, discovery, and consumer content. With millions of monthly users globally and a product used daily across multiple markets, the business is continuing to invest heavily into its backend engineering function.</p><p><br></p><p>They’re looking for a highly entrepreneurial Staff Software Engineer to help scale core backend systems while shaping technical direction across the platform. This role is ideal for someone who enjoys operating with autonomy, solving complex scaling challenges, and staying deeply hands-on.</p><p><br></p><p><strong>The environment particularly suits engineers who have previously been:</strong></p><ul><li><strong>Founding Engineers</strong></li><li><strong>Startup CTOs or early technical hires</strong></li><li><strong>Builders of side projects/products at scale</strong></li><li><strong>Engineers who thrive in fast-moving, high-ownership cultures</strong></li></ul><p><br></p><p>What You’ll Be Doing</p><ul><li>Designing and scaling backend services powering a high-traffic consumer platform</li><li>Leading architecture decisions across distributed systems and APIs</li><li>Remaining hands-on with coding, reviews, debugging, and optimisation</li><li>Partnering closely with Product, Data, Mobile, and Frontend teams</li><li>Driving engineering best practices across performance, reliability, and scalability</li><li>Mentoring engineers and helping elevate technical standards across the business</li></ul><p><br></p><p>Experience across several of the following is expected:</p><ul><li>Golang, Python, or Node.js</li><li>REST APIs, GraphQL, event-driven systems</li><li>GCP / cloud-native infrastructure</li><li>Microservices or modular monolith architectures</li><li>SQL & NoSQL databases</li><li>CI/CD and infrastructure-as-code</li><li>Monitoring, observability, and production systems</li></ul><p><br></p><p>Why Join?</p><ul><li>High-impact engineering role within a globally scaling platform</li><li>Strong product and engineering culture</li><li>Real ownership and influence on technical direction</li><li>Fast-moving environment with startup mentality at scale</li><li>Hybrid working model based in Berlin</li></ul>

Calculate Moving Average	SQL	Easy
Predict Customer Churn	Machine Learning	Medium
A/B Test Significance	Statistics	Medium
Optimize Query Performance	SQL	Hard
Feature Importance Analysis	Machine Learning	Medium
Clean Missing Data	Python	Easy
Neural Network Architecture	Deep Learning	Hard
Calculate Cohort Retention	SQL	Medium
Bayesian Probability	Statistics	Easy
Recommend Similar Products	Machine Learning	Hard

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