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‎.gitignore

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Hints.txt
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script.py

‎Basic Aggregate Functions/02- Average Selling Price/README.md

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# [Average Selling Price](https://leetcode.com/problems/average-selling-price/description/?envType=study-plan-v2&envId=top-sql-50)
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### Problem Requirements:
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Find the average selling price for each product. <code>average_price</code> should be rounded to 2 decimal places.
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Return the result table in any order
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<details>
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<summary style="font-size:1.3rem;"> <strong>Hints</strong> </summary>
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<details>
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<summary>Hint#1</summary>
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<p>Use <code>OUTER JOIN</code></p>
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</details>
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<details>
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<summary>Hint#2</summary>
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<p>JOIN ON <code>product_id </code>AND <code>purchase_date</code></p>
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</details>
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<details>
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<summary>Hint#3</summary>
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<p>SQL has an aggregation function called <code>SUM(expression)</code> which calculate the sum of values in a set</p>
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</details>
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<details>
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<summary>Hint#4</summary>
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<p>SQL has a <code>ROUND(number, decimals)</code> function which rounds a number to a specified number of decimal places. </p>
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</details>
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<details>
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<summary>Hint#5</summary>
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<p>SQL has a <code>COALESCE(val1 , val2 , ....)</code> function which Return the first non-null value in a list </p>
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</details>
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<details>
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<summary>Hint#6</summary>
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<p>Use <code>GROUP BY</code> clause to group a set of rows into a set of summary rows</p>
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</details>
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</details>
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<details>
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<summary style="font-size:1.3rem;"> <strong>Explanation</strong> </summary>
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To find the average_price for each product we want should calculate the total money paid and the total units bought then divide them as follows
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$$
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round(\frac{SUM(price \times units)}{SUM(units)} , 2)
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$$
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we will <code> JOIN</code> these tables <code> ON</code> <code> product_id </code> and <code> purchase_date</code>. The <code>purchase_date</code> should be between <code>start_date</code> and <code>end_date</code>.
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<ul>
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<li>To calculate the total money we can use <code>SUM()</code> function to sum the money of each record. The money of each record equals the <code>price</code>multiplied by <code>units</code>. </li>
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<li>To calculate the total units we also can use <code>SUM()</code> function </li>.
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<li>To calculate the <code>average_price</code> we should divide total money by total units then use <code>ROUND(number , decimals)</code> to round the result. </li>
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</ul>
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But what if the total units is zero here we will have <code>null</code>. To solve this problem we can use <code>COALESCE()</code> function. This function will return the first non-null value in a list.
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</details>
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<details>
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<summary style="font-size:1.3rem"><strong> SQL Solution</strong> </summary>
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```sql
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SELECT
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Prices.product_id,
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COALESCE(
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ROUND(
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SUM(Prices.price * UnitsSold.units) / SUM(UnitsSold.units),
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2
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),
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0
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) AS average_price
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FROM
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Prices
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LEFT OUTER JOIN UnitsSold ON Prices.product_id = UnitsSold.product_id
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AND UnitsSold.purchase_date >= Prices.start_date
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AND UnitsSold.purchase_date <= end_date
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GROUP BY
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product_id
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```
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</details>

‎Basic Aggregate Functions/04- Percentage of Users Attended a Contest/README.md

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# [Percentage of Users Attended a Contest](https://leetcode.com/problems/percentage-of-users-attended-a-contest/description/?envType=study-plan-v2&envId=top-sql-50)
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### Problem Requirements:
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Find the percentage of the users registered in each contest rounded to <strong>two decimals</strong>.
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Return the result table ordered by <code>percentage</code> in <strong>descending order</strong>. In case of a tie, order it by <code>contest_id</code> in <strong>ascending order</strong>.
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<details>
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<summary style="font-size:1.3rem;"> <strong>Hints</strong> </summary>
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<details>
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<summary>Hint#1</summary>
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<p>SQL has an aggregation function called <code>COUNT(expression)</code> which count all the rows that satisfy a specified condition</p>
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</details>
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<details>
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<summary>Hint#2</summary>
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<p>Use <code>subquery</code></p>
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</details>
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</details>
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<details>
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<summary style="font-size:1.3rem;"> <strong>Explanation</strong> </summary>
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To find the percentage of the users registered in each contest we should count the number of user that have registered into a contest and the total number of users then divide.
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<br>
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<br>
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<ul>
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<li>
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calculate the number of users that registered in each contest using <code>COUNT()</code> function.
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</li>
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<li>
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calculate the total number of user using <code>COUNT()</code> function as a <code><strong>sub query</strong></code>. By using <code>*</code> <i>(means all)</i> we can count the number of records in <code>Users</code> table.
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</li>
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<li>
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divide both number and multiply them by <code>100</code> Then use <code>ROUND(number , decimals)</code> to round the result.
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</li>
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</ul>
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</details>
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<details>
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<summary style="font-size:1.3rem"><strong> SQL Solution</strong> </summary>
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```sql
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SELECT
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contest_id,
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ROUND(
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COUNT(user_id)/(
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SELECT
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COUNT(user_id)
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FROM
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Users
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)* 100
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, 2) AS percentage
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FROM
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Register
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GROUP BY
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contest_id
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ORDER BY
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percentage DESC,
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contest_id
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```
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</details>

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