🧼 SQL WITH RECURSIVE – Master Tree & Hierarchy Queries

🧲 Introduction – Why Learn Recursive CTEs?

Ever needed to query a hierarchical dataset—like organizational charts, category trees, or file systems? Traditional SQL can struggle here. Enter WITH RECURSIVE, a powerful extension of Common Table Expressions (CTEs) that allows SQL queries to call themselves recursively.

This feature lets you navigate hierarchical or tree-structured data elegantly—without stored procedures or client-side loops.

🎯 In this guide, you’ll learn:

• What WITH RECURSIVE does and when to use it
• Syntax and structure
• Real-world examples
• Performance considerations and tips

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🔑 1. What Is WITH RECURSIVE in SQL?

WITH RECURSIVE is a feature that allows a Common Table Expression (CTE) to refer to itself. It’s most commonly used for:

• Tree traversal (e.g., parent-child)
• Graph algorithms (e.g., shortest path)
• Building paths or hierarchies

📘 Supported in:

• PostgreSQL
• MySQL 8+
• SQL Server (using CTE without RECURSIVE keyword but same structure)

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📚 2. Syntax of Recursive CTE

WITH RECURSIVE cte_name (column_list) AS (
  -- Anchor member (base query)
  SELECT ...

  UNION ALL

  -- Recursive member (calls itself)
  SELECT ... FROM cte_name
  JOIN ...
)
SELECT * FROM cte_name;

✅ Explanation:

• Anchor member: Base case (e.g., top-level employee)
• Recursive member: Refers to CTE itself to walk down the hierarchy
• UNION ALL combines the results

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🌳 3. Real-World Example: Employee Hierarchy

🗃️ Sample employees Table

id	name	manager_id
1	CEO	NULL
2	CTO	1
3	Dev1	2
4	Dev2	2
5	CFO	1

🔄 Query: Get Full Org Chart Starting from CEO

WITH RECURSIVE employee_hierarchy AS (
  SELECT id, name, manager_id, 1 AS level
  FROM employees
  WHERE manager_id IS NULL

  UNION ALL

  SELECT e.id, e.name, e.manager_id, eh.level + 1
  FROM employees e
  JOIN employee_hierarchy eh ON e.manager_id = eh.id
)
SELECT * FROM employee_hierarchy;

✅ Explanation:

• Starts at the CEO (manager_id IS NULL)
• Joins each employee with their manager recursively
• Adds a level column to indicate hierarchy depth

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🧭 4. Recursive CTE with Path Generation

WITH RECURSIVE category_paths(id, name, path) AS (
  SELECT id, name, name AS path
  FROM categories
  WHERE parent_id IS NULL

  UNION ALL

  SELECT c.id, c.name, CONCAT(cp.path, ' > ', c.name)
  FROM categories c
  JOIN category_paths cp ON c.parent_id = cp.id
)
SELECT * FROM category_paths;

💡 Tip: Use string concatenation to build breadcrumb paths like Electronics > Computers > Laptops.

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🧪 5. Recursive Depth Limitation

PostgreSQL

WITH RECURSIVE limited_cte AS (
  SELECT ... , 1 as level
  FROM ...

  UNION ALL

  SELECT ..., level + 1
  FROM limited_cte
  WHERE level < 5
)

⚠️ Avoid infinite recursion with a level or depth limit clause.

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🧰 6. Use Cases of WITH RECURSIVE

Use Case	Description
🔍 Hierarchical data	Org charts, category trees
🔁 Graph traversal	Linked nodes, friends-of-friends
🧭 Breadcrumb paths	Web navigation paths
📈 Dependency resolution	Build order of packages/components
🪢 Bill of Materials (BOM)	Multi-level component breakdowns

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💡 Tips & ⚠️ Gotchas

💡 Best Practices	⚠️ Pitfalls to Avoid
Always include a recursion limit	Infinite loops can crash your session
Use UNION ALL not UNION unless deduping	UNION is slower due to duplicate checks
Add a LEVEL or DEPTH column	Helps debugging and limits traversal depth
Test anchor query and recursive part separately	Hard to debug if recursive logic is complex

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📌 Summary – Recap & Next Steps

WITH RECURSIVE enables powerful hierarchical and graph-based queries in SQL, allowing you to traverse parent-child data without writing procedural code.

🔍 Key Takeaways:

• Use a CTE with an anchor and recursive part.
• Combine results using UNION ALL.
• Always use depth limits to control recursion.
• Ideal for org charts, BOMs, and tree structures.

⚙️ Real-World Relevance:
Used in ERPs, file systems, organizational tools, and any application involving tree-like data structures.

❓ FAQ – SQL WITH RECURSIVE

❓ Is WITH RECURSIVE supported in all databases?

✅ Mostly:

• ✅ PostgreSQL, MySQL 8+, SQLite: Yes (with WITH RECURSIVE)
• ✅ SQL Server: Yes, structure is similar but no RECURSIVE keyword needed

❓ What’s the difference between CTE and Recursive CTE?

✅ Regular CTE runs once. Recursive CTE refers to itself and runs iteratively until no rows are returned from the recursive part.

❓ How do I stop infinite recursion?

✅ Use a condition like WHERE level < 10 or WHERE id IS NOT NULL in the recursive clause.

❓ Can I use recursion to replace loops?

✅ Yes. Recursive CTEs replace many procedural loops, especially for tree-like structures.

❓ What does UNION ALL do in recursion?

✅ It merges the anchor and recursive results together. ALL improves performance by skipping duplicate checks.

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