Which of the following statements correctly describes how subqueries work in SQL?

Introduction / Context:
Subqueries are an essential concept in SQL and are widely used in interview questions to check how well a candidate understands query nesting and query composition. A subquery is simply a query within another query, but there are many myths and misunderstandings about what subqueries can and cannot do. This question asks you to identify the one correct statement about subqueries among several misleading or incorrect statements.

Given Data / Assumptions:

• We are working with standard SQL that supports subqueries.
• The term subquery refers to a query nested inside another SQL statement.
• We must choose the statement that correctly describes subqueries.

Concept / Approach:
A subquery is typically written as a SELECT statement enclosed in parentheses that appears in the WHERE, FROM, or SELECT clause of an outer query. The main query that uses the subquery is called the outer query, and the nested query is called the inner query. Subqueries can return single values, multiple rows, or even entire result sets, and in many cases they can be rewritten as equivalent non nested queries using joins or other constructs.

Step-by-Step Solution:
Step 1: Evaluate option A, which claims that a subquery does not start with SELECT. In reality, almost all subqueries are SELECT statements, so this is false. Step 2: Evaluate option B, which states that a subquery involves an inner query nested inside an outer query. This correctly captures the basic structure of subqueries. Step 3: Evaluate option C, which claims that a subquery cannot return the same result as an equivalent non nested query. This is false because many subqueries can be rewritten as joins or single level queries. Step 4: Since only option B is true, option D, which says all statements are correct, must be false. Step 5: Conclude that option B is the only correct description of how subqueries work in SQL.

Verification / Alternative check:
You can verify the definition by writing a simple query such as SELECT name FROM employees WHERE department_id IN (SELECT id FROM departments WHERE location = 'London'). Here, the inner SELECT inside the parentheses is a subquery, and the outer SELECT uses its result. Many database textbooks and official documentation define subqueries in this same way, confirming that the inner and outer query relationship is the defining feature.

Why Other Options Are Wrong:
Option A is incorrect because subqueries almost always start with SELECT, and the idea that they do not start with SELECT directly contradicts standard SQL syntax. Option C is incorrect because subqueries can often be replaced by equivalent non nested queries using joins or grouping, which may return the same result set. Option D is incorrect because it asserts that all options are correct, but we have already identified that options A and C are not valid statements about subqueries.

Common Pitfalls:
Many learners believe that subqueries are always slower or more powerful than joins, but performance depends on the database engine and query structure. Another common mistake is misusing correlated subqueries, where the inner query refers to columns from the outer query and is executed repeatedly. Finally, some developers incorrectly think that subqueries must always return a single value, but subqueries can return multiple rows or even sets of rows depending on where they are used.

Final Answer:
The correct statement is that a subquery involves the use of an inner query that is nested inside an outer query in SQL.