A subquery, also known as an inner query or nested query, is a query that is embedded within another SQL statement. The purpose of using subqueries in SQL databases is to retrieve data from one or more tables and use it as input for another query.

Subqueries are often used to filter or manipulate data based on certain conditions, and they can be used in many different scenarios, such as:

Filtering results: A subquery can be used to filter results based on a specific condition. For example, you can use a subquery to find all the customers who have placed an order in the last 30 days.

Sorting results: Subqueries can be used to sort results based on a specific condition. For example, you can use a subquery to sort all the products in a category by their price.

Aggregating data: Subqueries can be used to aggregate data from multiple tables. For example, you can use a subquery to calculate the total sales for each customer.

Testing conditions: Subqueries can be used to test conditions and return a boolean value. For example, you can use a subquery to test whether a customer has made a purchase in the last 30 days.