Which statement best describes a nested subquery?

Which statement best describes a nested subquery?

• A. It is a subquery that runs independently of the outer query.
• B. It is a subquery contained within another subquery, where the outer query uses the inner results.
• C. It is a subquery that can only appear in the FROM clause.
• D. It is a subquery that always returns a single row.

Answer: (B)

A nested subquery is a subquery placed inside another query, where the outer query uses the results produced by the inner query. The inner query runs to generate a set of values (or a single value) that the outer query then relies on to complete its operation, such as filtering, matching, or computing. That’s why the description of a subquery contained within another subquery, with the outer query using the inner results, is the best fit. It captures the data flow: inner executes to supply data, outer uses that data to determine its results. For example, you might have a query that finds employees whose department ID appears in the set returned by an inner query that selects department IDs in a specific location. The inner query provides the list, and the outer query uses it to filter employees. Other statements don’t fit as well. A subquery that runs independently of the outer query isn’t truly nested, since there’s no dependency feeding the outer result. Subqueries aren’t limited to the FROM clause; they can appear in WHERE, SELECT, or HAVING as well. And subqueries aren’t restricted to returning a single row—they can return one value, many values, or even be used in a way that checks for existence, so they aren’t limited to a single-row result.

A nested subquery is a subquery placed inside another query, where the outer query uses the results produced by the inner query. The inner query runs to generate a set of values (or a single value) that the outer query then relies on to complete its operation, such as filtering, matching, or computing.

That’s why the description of a subquery contained within another subquery, with the outer query using the inner results, is the best fit. It captures the data flow: inner executes to supply data, outer uses that data to determine its results. For example, you might have a query that finds employees whose department ID appears in the set returned by an inner query that selects department IDs in a specific location. The inner query provides the list, and the outer query uses it to filter employees.

Other statements don’t fit as well. A subquery that runs independently of the outer query isn’t truly nested, since there’s no dependency feeding the outer result. Subqueries aren’t limited to the FROM clause; they can appear in WHERE, SELECT, or HAVING as well. And subqueries aren’t restricted to returning a single row—they can return one value, many values, or even be used in a way that checks for existence, so they aren’t limited to a single-row result.