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Subqueries, often referred to as nested queries, are a powerful feature in PostgreSQL that allows you to create complex SQL queries by nesting one query inside another. In this guide, we’ll embark on a journey to unlock the true potential of subqueries, starting from the basics and progressing to more advanced techniques.
Section 1: Understanding Subqueries
Before we dive into the technical details, it’s crucial to understand the concept of subqueries. A subquery is a query that is embedded inside another query. It can be used to retrieve data that will be used by the main query. Let’s start by looking at a simple example:
In this example, the inner subquery `(SELECT department_id FROM departments WHERE department_name = ‘Sales’)` retrieves the department ID for the ‘Sales’ department, which is then used by the outer query to fetch employee names in that department.
