Friday, March 18, 2016

🧠 20 Common Table Expression (CTE) Interview Questions You Must Know




Published on: June 7, 2025

Category: SQL / Database

Tags: SQL, CTE, Interview Questions, SQL Server, Common Table Expressions, Recursive Queries, Performance Tuning, .NET Dev Corner



🔍 What is a Common Table Expression (CTE)?

A Common Table Expression (CTE) is a temporary result set defined within the execution scope of a SELECT, INSERT, UPDATE, or DELETE statement. It’s often used to simplify complex joins, subqueries, or recursive logic.



📋 CTE Interview Questions — With Increasing Difficulty



🔹 Beginner-Level

  1. What is a Common Table Expression (CTE) in SQL? – Define the purpose and use cases.
  2. How is a CTE different from a subquery or derived table? – Discuss readability and reusability.
  3. What are the basic syntax rules for writing a CTE? – E.g. WITH CTE_Name AS (...)
  4. Can a CTE be used multiple times in the same query?
  5. What are the advantages of using a CTE over temporary tables? – E.g. memory management, readability.


🔹 Intermediate-Level

  1. How do you use a CTE to improve query readability?
  2. How can you join a CTE with another table? – Provide syntax example with JOIN.
  3. What happens if you alias a CTE the same as a table name?
  4. Can a CTE reference itself or be recursive? Give an example.
  5. What is the scope of a CTE — how long does it exist in memory?
  6. Can you use multiple CTEs in a single query? How? – Demonstrate using comma-separated CTEs.
  7. Can a CTE contain an ORDER BY clause? If yes, in what context?


🔹 Advanced-Level / Real-World Scenarios

  1. How do you write a recursive CTE to flatten a hierarchical structure (e.g., employee-manager)?
  2. How do you limit recursion depth in a recursive CTE?
  3. Compare performance: CTE vs. temporary table vs. subquery.
  4. Can you use a CTE to update or delete records? How?
  5. Can CTEs be used in stored procedures or views?
  6. Explain the use of CTEs in pagination queries.
  7. What is the difference between recursive CTE and loop-based logic?
  8. Why might a CTE slow down performance, and how do you troubleshoot it?


🧪 Bonus Tip

You can view the SQL generated by a CTE using tools like:

  • SQL Server Execution Plans
  • SET STATISTICS IO ON
  • EXPLAIN in PostgreSQL
  • ToQueryString() in EF Core


✅ Wrap-Up

CTEs are incredibly powerful tools in SQL. Whether you're preparing for interviews or trying to write better queries, mastering them can give you an edge in performance, readability, and maintainability.



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