**Course Title:** SQLite Mastery: Lightweight Database Management **Section Title:** Subqueries and Advanced Data Retrieval **Topic:** Understanding subqueries and their use cases **Introduction** In the previous topics, we explored various techniques for querying and manipulating data in SQLite databases. However, there are situations where we need to perform more complex queries that involve nesting one query inside another. This is where subqueries come in. In this topic, we will delve into the world of subqueries, exploring what they are, how they work, and their use cases. **What are subqueries?** A subquery is a query nested inside another query. The inner query is used to retrieve data that is then used by the outer query. Subqueries can be used in various parts of a query, such as in the WHERE clause, FROM clause, or even as a select list. **Types of subqueries** There are two main types of subqueries: 1. **Scalar subqueries**: These subqueries return a single value, which can then be used in the outer query. 2. **Table subqueries**: These subqueries return a set of rows, which can then be used in the outer query. **Use cases for subqueries** Subqueries are useful in a variety of situations, including: 1. **Comparing data**: Subqueries can be used to compare data between two or more tables. 2. **Filtering data**: Subqueries can be used to filter data based on conditions that are not directly accessible in the outer query. 3. **Performing calculations**: Subqueries can be used to perform calculations that involve data from multiple tables. **Example: Using a subquery to filter data** Let's consider an example where we have two tables: `orders` and `customers`. We want to retrieve all orders from customers who have a balance greater than $1000. We can use a subquery to achieve this: ```sql SELECT * FROM orders WHERE customer_id IN ( SELECT customer_id FROM customers WHERE balance > 1000 ); ``` In this example, the subquery retrieves the customer IDs from the `customers` table where the balance is greater than $1000. The outer query then uses these customer IDs to retrieve the corresponding orders from the `orders` table. **Example: Using a subquery to perform calculations** Let's consider an example where we have two tables: `orders` and `products`. We want to retrieve the total value of all orders for each product category. We can use a subquery to achieve this: ```sql SELECT category, SUM(total) AS total_value FROM ( SELECT category, SUM(quantity * price) AS total FROM orders GROUP BY category ) AS subquery; ``` In this example, the subquery retrieves the total value of all orders for each product category by joining the `orders` table with the `products` table. The outer query then uses the subquery to retrieve the total value for each category. **Best practices for using subqueries** When using subqueries, keep the following best practices in mind: 1. **Use subqueries sparingly**: Subqueries can improve readibility and make your code more efficient, but they can also decrease performance if not used judiciously. 2. **Optimize your subqueries**: Make sure to optimize your subqueries to reduce the amount of data being retrieved and processed. 3. **Use aliases**: Use aliases to make your subqueries more readable and easier to understand. **Conclusion** In this topic, we explored the world of subqueries, including what they are, how they work, and their use cases. We also saw examples of how to use subqueries to filter data and perform calculations. By mastering subqueries, you can take your SQLite skills to the next level and tackle more complex queries with ease. **What's next?** In the next topic, we will explore how to write scalar and table subqueries in more depth. We will also cover advanced techniques for using subqueries to improve your queries. **Leave a comment or ask for help** If you have any questions or need help with subqueries, feel free to leave a comment below. We'll do our best to help you master subqueries and improve your SQLite skills. **Additional resources** * SQLite documentation: [https://www.sqlite.org/lang_select.html](https://www.sqlite.org/lang_select.html) * Subquery tutorial by Tutorials Point: [https://www.tutorialspoint.com/sqlite/sqlite_subqueries.htm](https://www.tutorialspoint.com/sqlite/sqlite_subqueries.htm)