Recursive CTEs in MySQL: The Definitive Guide

What Is a Recursive CTE?

A recursive Common Table Expression (CTE) lets you query hierarchical data—such as organizational charts, folder trees, bill of materials, or graph networks—by repeatedly executing the same query block until a termination condition is met. Introduced in MySQL 8.0, recursive CTEs are the ANSI-SQL-standard alternative to older vendor-specific features like CONNECT BY in Oracle or the dreaded “adjacency list + while loop” pattern in pre-8.0 MySQL.

Why Recursive CTEs Matter

Modern applications generate complex, interconnected datasets: think comment threads, category trees, and multi-level referrals. Modeling those relationships is straightforward—querying them efficiently used to be anything but. Before MySQL 8.0 you had to rely on stored procedures, temporary tables, or recursive logic in your application layer. Recursive CTEs pull that heavy lifting back into the database engine, giving you:

• Simplicity: Express an entire tree traversal in a single SQL statement.
• Maintainability: Stick to ANSI-SQL and avoid proprietary hacks.
• Performance: Let the optimizer choose smart execution plans and memory limits.
• Security: Keep recursion inside the database—no round trips, no SQL injection risk from ad-hoc loops.

How Recursive CTEs Work in MySQL

Every recursive CTE has two parts:

1. Anchor member – Runs once to seed the recursion.
2. Recursive member – References the CTE itself, runs repeatedly, and must converge (terminate) via a WHERE clause or exhausted rows.

MySQL repeatedly unions the results (UNION ALL by default) until the recursive member returns zero rows or until you hit max_execution_time or cte_max_recursion_depth (default 1000).

Basic Syntax

WITH RECURSIVE cte_name (col1, col2, ...) AS (
-- Anchor member
anchor_query
UNION ALL
-- Recursive member
recursive_query referencing cte_name
)
SELECT * FROM cte_name;


Practical Example: Employee Org Chart

Assume a classic employees table:

CREATE TABLE employees (
emp_id INT PRIMARY KEY,
emp_name VARCHAR(50),
manager_id INT NULL,
FOREIGN KEY (manager_id) REFERENCES employees(emp_id)
);


We want an ordered list of all direct and indirect reports under a given manager (say, emp_id = 3):

WITH RECURSIVE org_path (emp_id, emp_name, manager_id, lvl) AS (
-- Anchor: start at the chosen manager
SELECT emp_id, emp_name, manager_id, 0 AS lvl
FROM employees
WHERE emp_id = 3

UNION ALL

-- Recursive: find subordinates of the previous level
SELECT e.emp_id, e.emp_name, e.manager_id, p.lvl + 1
FROM employees e
JOIN org_path p ON e.manager_id = p.emp_id
)
SELECT *
FROM org_path
ORDER BY lvl, emp_id;


The result is the complete managerial tree in depth-first order—no temp tables, no loops.

Performance & Best Practices

1. Index Appropriately

The recursive member often joins child rows to parent rows. A B-Tree index on the foreign-key column (manager_id in the example) is critical.

2. Keep Columns Minimal

Only select what you actually need. Recursive CTEs materialize intermediate rows; extra columns consume memory.

3. Avoid Cycles

A mis-inserted row can create a self-loop (manager_id = emp_id) and lead to infinite recursion. Use application logic, triggers, or a dedicated CHECK to disallow cyclic references.

4. Use cte_max_recursion_depth Safely

Default depth 1000 is usually enough, but for very deep graphs you may raise it. Just ensure your data really contains that many levels and you’re not masking a bug.

5. Test on Realistic Data Volumes

Recursion multiplies rows quickly. EXPLAIN, monitor Sort_merge_passes, and set an upper bound with MAXRECURSION (MariaDB) or by rewriting with LIMIT/OFFSET if feasible.

Common Misconceptions

• “Recursive CTEs are slower than loops.” Not necessarily. MySQL can optimize set-based recursion better than row-by-row client loops, especially with proper indexing.
• “UNION, not UNION ALL, is safer.” Using UNION forces a costly DISTINCT on every iteration. Stick to UNION ALL unless duplicates truly break your business logic.
• “I can’t paginate recursive results.” You can—wrap the outer SELECT in a subquery and apply LIMIT/OFFSET. Just be mindful that the whole CTE still materializes before limiting.

Galaxy and Recursive CTEs

Galaxy’s modern SQL editor recognizes recursive CTE patterns and provides context-aware autocomplete for anchor vs. recursive members, highlights potential cycles, and suggests adding an index on the recursive key. With Galaxy’s AI Copilot, you can even prompt “Generate a recursive CTE to flatten my categories table” and receive a verified query plus inline documentation—all without leaving the editor.

Real-World Use Cases

Materialized Path Rebuilding

Migrate from an adjacency list to a materialized path by walking the tree and concatenating keys.

Bill of Materials (BOM)

Compute nested components and aggregated costs per level for manufacturing systems.

Graph Shortest Path

Combine recursive CTEs with window functions to compute hop counts in an unweighted network.

Putting It All Together

Recursive CTEs transform once-painful hierarchical queries into concise, readable, ANSI-compliant SQL. Master them and you’ll write faster, safer, and more expressive queries—especially in a tool like Galaxy that supercharges the workflow with AI suggestions and collaboration.