Recursive CTE in SQL: Syntax and Use Cases
💡 Concept Name
Recursive Common Table Expression (CTE) – A special type of CTE that calls itself repeatedly to return hierarchical or sequential data.
📘 Quick Intro
Recursive CTEs allow SQL to handle hierarchical relationships like employee-manager chains or category trees. They work by combining an anchor query and a recursive query with a UNION ALL.
🧠 Analogy / Short Story
Think of recursive CTEs like climbing a family tree: you start with a single person (anchor query) and move upward through generations (recursive part) until there’s no one left to trace. SQL builds this tree step by step using recursion.
🔧 Technical Explanation
- A recursive CTE must contain two parts: anchor query and recursive member.
- The anchor query provides the starting point (e.g., root node).
- The recursive member repeatedly joins the CTE to itself.
- The recursion stops when the recursive part returns no rows.
- Syntax includes a
WITH RECURSIVEclause (PostgreSQL, SQL Server, etc.).
💻 Real Code Example
-- Example: Find employee hierarchy
WITH RECURSIVE EmployeeCTE AS (
SELECT id, name, manager_id, 1 AS level
FROM employees
WHERE manager_id IS NULL -- Start with top-level manager
UNION ALL
SELECT e.id, e.name, e.manager_id, c.level + 1
FROM employees e
INNER JOIN EmployeeCTE c ON e.manager_id = c.id
)
SELECT * FROM EmployeeCTE;
🎯 Purpose & Use Case
- ✅ Traverse organizational hierarchies like employee-manager structures.
- ✅ Process category trees in e-commerce systems.
- ✅ Generate sequences or calculate factorials/fibonacci values.
- ✅ Track dependency chains in configuration or tasks.
❓ Interview Q&A
Q1: What is a recursive CTE?
A: A CTE that references itself to retrieve hierarchical or repeating data structures.
Q2: What are the two main components of a recursive CTE?
A: The anchor member and the recursive member.
Q3: When does the recursion stop?
A: When the recursive query returns no new rows to process.
Q4: What SQL clause is used to define a recursive CTE?
A: WITH RECURSIVE in PostgreSQL or just WITH in SQL Server.
Q5: Can recursive CTEs cause infinite loops?
A: Yes, if the termination condition is incorrect or missing.
Q6: Are recursive CTEs supported in MySQL?
A: Yes, starting from MySQL 8.0.
Q7: What is a common real-world use of recursive CTEs?
A: Employee-manager hierarchy traversal.
Q8: How can you limit recursion depth?
A: Use an extra column like 'level' and a WHERE clause to cap it.
Q9: Can recursive CTEs return sorted results?
A: Yes, with an ORDER BY in the final SELECT.
Q10: What makes recursive CTEs better than self-joins?
A: They are more readable, scalable, and better for deep hierarchies.
📝 MCQs
Q1. What is required for a recursive CTE?
- Just a SELECT
- Stored procedure
- Anchor and recursive member
- Loop statement
Q2. What keyword starts a recursive CTE?
- CREATE CTE
- WITH
- WITH RECURSIVE
- BEGIN CTE
Q3. When does a recursive CTE stop?
- After 100 rows
- After 10 levels
- When no new rows are returned
- After 1 join
Q4. Which structure is ideal for recursive CTEs?
- Flat tables
- Hashed indexes
- Hierarchical data
- Procedural tasks
Q5. What clause combines anchor and recursion?
- JOIN
- MERGE
- UNION ALL
- EXCEPT
Q6. Recursive CTEs are more readable than?
- Functions
- Triggers
- Self-joins
- Views
Q7. What SQL Server version supports recursive CTEs?
- SQL Server 2000
- SQL Server 2019+
- SQL Server 2005+
- Not supported
Q8. What can prevent infinite recursion?
- ROLLBACK
- LIMIT 1
- Termination condition
- Group By
Q9. Recursive CTEs in PostgreSQL use which clause?
- SELECT RECURSIVE
- CTE RECURSIVE
- WITH RECURSIVE
- UNION RECURSIVE
Q10. What use case fits recursive CTEs best?
- Simple counts
- Foreign key checks
- Organizational hierarchy
- Backup logs
💡 Bonus Insight
Recursive CTEs are powerful for tree and graph traversal. Use safeguards like MAXRECURSION (in SQL Server) or recursion limits to avoid runaway queries in production environments.
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