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<p>An online marketplace company has introduced a new feature that allows potential buyers and sellers to conduct audio chats with each other prior to transacting.</p>

<p>Let’s say we have two tables that represent this data.</p>

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

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

<p><code>chats</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>buyer_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>seller_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>call_length</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>call_connected</code></td>
<td>INTEGER</td>
</tr>
</tbody>
</table>
<p><code>marketplace_purchases</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>buyer_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>seller_user_id</code></td>
<td>INTEGER</td>
</tr>

<tr>
<td><code>item_id</code></td>
<td>INTEGER</td>
</tr>

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

<ol>
<li>How would you measure the success of this new feature?</li>
<li>Write a query that can represent if the feature is successful or not.</li>
</ol>


How would you measure the success of an online marketplace introducing an audio chat feature given a dataset of their usage?

Audio Chat Success

<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>Let’s say that your company is running a standard control and variant AB test on a feature to increase conversion rates on the landing page. The PM checks the results and finds a .04 p-value.</p>

<p>How would you assess the validity of the result?</p>


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