A subquery inside a subquery is called?
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of SQL & Databases conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
A subquery placed inside another subquery is called a nested subquery (or multi-level subquery):
SELECT customer_name
FROM customers
WHERE customer_id IN (
SELECT customer_id
FROM orders
WHERE order_id IN (
SELECT order_id
FROM order_items
WHERE product_id = 101
)
);
While nested subqueries are intuitive, deeply nested subqueries can degrade execution plan efficiency. Modern optimizers often rewrite them into JOIN operations, or developers rewrite them into Common Table Expressions (CTEs) using WITH.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.