Writing Recursive CTEs in MySQL

Overview

Recursive common-table expressions (CTEs) were introduced in MySQL 8.0.1 and finally gave MySQL developers a standards-compliant way to traverse hierarchical data, generate running totals, and solve problems that previously required stored procedures or application-side loops. By declaring a WITH RECURSIVE query, you can define an anchor member (your starting rows) and a recursive member (the part that calls itself). MySQL automatically iterates until no new rows are produced or a loop-prevention limit is hit.

Why Recursive CTEs Matter

Hierarchical data is everywhere: org charts, folder structures, bill-of-materials, dependency graphs, network hops—the list goes on. Recursive CTEs let you handle these patterns entirely in SQL, keeping business logic close to the data and minimizing round-trips. For data engineers and analysts, this means:

• Cleaner, more maintainable SQL compared with self-joins or user-defined variables.
• Better performance thanks to set-based execution inside the optimizer.
• Portability across PostgreSQL, SQL Server, Oracle, and now MySQL.

Basic Syntax

WITH RECURSIVE cte_name (col_list) AS (
-- Anchor member
SELECT ...
UNION ALL
-- Recursive member
SELECT ... FROM cte_name JOIN ...
)
SELECT * FROM cte_name;


The rules:

• Anchor comes first and must not reference the CTE.
• Recursive member must reference the CTE exactly once.
• UNION ALL is required (the ANSI spec allows UNION, but MySQL insists on ALL).
• Column list is optional but recommended for clarity.
• MySQL limits recursion to 1000 iterations by default; override with SET @@cte_max_recursion_depth = n;

Step-by-Step Example: Employee Hierarchy

Suppose you have an employees table with columns emp_id, emp_name, and manager_id. You want the full reporting chain for employee 7.

-- Anchor: start with the employee
WITH RECURSIVE hierarchy AS (
SELECT emp_id, emp_name, manager_id, 0 AS depth
FROM employees
WHERE emp_id = 7

UNION ALL

-- Recursive: add the manager each loop
SELECT e.emp_id, e.emp_name, e.manager_id, h.depth + 1
FROM employees AS e
JOIN hierarchy AS h ON e.emp_id = h.manager_id
)
SELECT * FROM hierarchy;


The query returns the employee, her manager, the manager’s manager, and so on, until manager_id becomes NULL. The depth column shows hierarchy level and is handy for ordering:

SELECT * FROM hierarchy ORDER BY depth;

Generating Numbers on the Fly

Need a quick tally table for a date dimension or pagination? Use a recursive CTE:

WITH RECURSIVE nums AS (
SELECT 1 AS n
UNION ALL
SELECT n + 1 FROM nums WHERE n < 1000
)
SELECT n FROM nums;

You now have integers 1–1000 without creating a physical table.

Performance & Optimization

1. Add Indexes to Join Columns

Recursive members usually join the CTE back to the base table. Make sure columns such as manager_id are indexed.

2. Limit the Result Set Early

Filter the anchor or recursive member to avoid ballooning data. Even a simple WHERE depth < 10 can prevent runaway recursion.

3. Watch Recursion Depth

If the data contains cycles (e.g., an employee managing herself), the query may exceed cte_max_recursion_depth. Clean the data or add a break condition.

Best Practices

• Explicit column lists guard against mismatched anchor/recursive projections.
• UNION ALL vs UNION: stick to ALL to avoid the costly deduplication step.
• Order results outside the CTE; internal ORDER BY disrupts recursion and is disallowed in MySQL.
• Comment generously; recursive logic is powerful but opaque to newcomers.

Common Pitfalls

Infinite Loops

Forgetting a termination condition can exceed the recursion depth. Always ensure the recursive member shrinks the problem space.

Mismatched Columns

The anchor and recursive queries must project the exact number and order of columns. MySQL throws 1248: Every select must have the same number of columns.

Using UNION Instead of UNION ALL

MySQL will error out; switch to UNION ALL.

Writing Recursive CTEs in Galaxy

Galaxy’s modern SQL editor provides syntax highlighting, auto-completion, and an AI copilot that recognizes recursion patterns. Type WITH RECURSIVE and Galaxy auto-suggests anchor/recursive scaffolding. You can:

• Rename columns on the fly and get live warnings if the anchor and recursive projections don’t match.
• Share the CTE in a Collection so teammates can reuse a verified hierarchy query.
• Ask the AI copilot to "optimize this recursive CTE"; it may propose indexing strategies or depth limits.

Because Galaxy runs queries natively against MySQL, execution plans and run-time metrics appear in a panel, helping you fine-tune recursion depth and indexes without leaving the editor.

Conclusion

Recursive CTEs unlock elegant, set-based solutions for hierarchical queries in MySQL 8+. By understanding the anchor-recursive pattern, guarding against run-away loops, and leveraging tools like Galaxy, you can write maintainable SQL that stays performant—even as your data grows.