Subquery depth: can SQL subqueries be nested multiple levels deep (subject to practical and vendor limits)?

Introduction / Context:
Subqueries are SELECT statements embedded inside another SQL statement. They are used for filtering, deriving values, and correlating results. This question asks whether subqueries can be nested multiple times.

Given Data / Assumptions:

• SQL supports subqueries in clauses such as SELECT, FROM, WHERE, and HAVING (vendor-specific variations apply).
• Vendors may impose recursion or depth limits, but these are generally generous.
• Readability and performance, not syntax, are the usual constraints.

Concept / Approach:
Nesting subqueries allows stepwise problem solving: a subquery computes an intermediate set, which another query further filters or aggregates. Correlated subqueries reference columns from the outer query. While nesting is allowed, many problems are clearer and faster using JOINs or Common Table Expressions (CTEs).

Step-by-Step Solution:

Verification / Alternative check:
Convert deep nesting to CTEs for readability; execution plans should be functionally equivalent.

Why Other Options Are Wrong:

• Incorrect / “Only one/two levels”: SQL syntax permits more than one level; limits are practical.
• “Only allowed in WHERE”: Scalar and derived table subqueries are valid in other clauses.

Common Pitfalls:
Overusing correlated subqueries causing performance issues; ignoring indexes; not analyzing execution plans.

Final Answer:
Correct