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A subquery in SQL, also known as a nested query or inner query, is a query that is embedded within another SQL query. It is a powerful SQL feature that allows you to use the result of one query as a part of another query. Subqueries are commonly used to retrieve or manipulate data in a more complex and selective manner.
I. Key Characteristics of Subqueries:
1. Syntax:
Subqueries are enclosed within parentheses and are usually placed within the WHERE clause of the outer query. They can be used in various parts of a SQL statement, such as the SELECT, INSERT, UPDATE, or DELETE statements.
2. Purpose:
Subqueries are used to perform operations like filtering, sorting, or aggregating data based on the result of another query. They are often used to answer questions like “Which rows in one table are related to certain rows in another table?” or “What is the maximum/minimum value of a particular column within a subset of data?”
3. Single or Multiple Rows:
Subqueries can return a single value, a single row, or multiple rows of data, depending on the specific query. The type of subquery depends on the operator used (e.g., =, IN, ANY, ALL, etc.) and the context in which it’s used.
