In SQL, what is a subquery in a SELECT statement and how is it typically used within another query?

Introduction / Context:
Subqueries are an important concept in SQL used extensively in Oracle, SQL Server, and other relational databases. They allow you to break complex logic into smaller pieces by embedding one query inside another query. For certification exams and practical database work, you must clearly understand what a subquery is and how it is used in SELECT statements and other SQL commands.

Given Data / Assumptions:

• The question is about the definition and typical use of a subquery.
• We are considering standard SQL syntax used in major relational database systems.
• The context is a SELECT statement, although subqueries can appear in INSERT, UPDATE, and DELETE as well.
• No specific table structure or numeric computation is provided, so the focus is conceptual.

Concept / Approach:
A subquery is simply a query within a query. Technically, it is an inner SELECT statement whose result is passed to an outer statement. Subqueries can appear in different parts of an SQL command: in the WHERE clause to filter rows, in the FROM clause as an inline view, or in the SELECT list to compute derived values. The outer query can then use the inner query's result set to refine its own result. Understanding that a subquery is nested and supportive to the main query is the key idea tested here.

Step-by-Step Solution:
Step 1: Recall that SQL allows nesting of SELECT statements, where one SELECT appears inside the parentheses of another SQL statement. Step 2: Recognize that the inner SELECT is called a subquery, and the surrounding statement is the outer or main query. Step 3: Note that the subquery returns a set of rows or a single value that the outer query uses for filtering, comparison, or as a derived table. Step 4: Match this definition with the answer choices, selecting the one that correctly describes a subquery as a nested SELECT used by the outer query.

Verification / Alternative check:
Consider an example: SELECT * FROM employee WHERE salary > (SELECT AVG(salary) FROM employee). The inner query calculates the average salary, and the outer query returns employees whose salary is greater than that average. Here, (SELECT AVG(salary) FROM employee) is a subquery embedded in the WHERE clause. This concrete example confirms that the correct description involves a nested SELECT providing a result to an outer query.

Why Other Options Are Wrong:
Option B is wrong because a subquery is not a stored procedure; it is simply an inner SELECT. Option C is incorrect because subqueries do not automatically update rows; they only return results. Option D is incorrect because subqueries can appear not only in the WHERE clause but also in the FROM and SELECT clauses and in other SQL statements like INSERT, UPDATE, and DELETE depending on the database.

Common Pitfalls:
A common mistake is to confuse subqueries with joins. While joins combine tables horizontally in a single query block, subqueries first compute a result and then feed it to the outer query. Another pitfall is assuming that subqueries are always slower; in many optimizers, subqueries can be transformed into joins internally. Understanding where subqueries can appear and how they interact with the outer query helps you write clear and maintainable SQL code.

Final Answer:
A subquery is a SELECT statement nested inside another SQL statement to provide a value set or result used by the outer query.