Recursive CTE in MySQL 8: What It Is and How It Works

A recursive CTE in MySQL 8 is a Common Table Expression that calls itself to repeatedly process rows until a condition is met. It helps handle hierarchical or iterative data queries, like finding all subordinates in an employee tree, using a simple, readable SQL structure.

How It Works

A recursive CTE works like a loop inside a SQL query. Imagine you want to find all the members in a family tree starting from one person. The recursive CTE starts with a base query that selects the first person (the anchor). Then it repeatedly runs a recursive part that finds the next level of family members related to the previous results.

This process continues until no new rows are found, similar to how you might explore a tree branch by branch. The recursive CTE combines these results into one table, making it easy to query hierarchical or connected data without complex programming.

Example

This example shows a simple employee hierarchy. The recursive CTE finds all employees under a manager with ID 1.

sql

WITH RECURSIVE EmployeeHierarchy AS (
  SELECT id, name, manager_id
  FROM employees
  WHERE id = 1  -- Start from manager with ID 1
  UNION ALL
  SELECT e.id, e.name, e.manager_id
  FROM employees e
  INNER JOIN EmployeeHierarchy eh ON e.manager_id = eh.id
)
SELECT * FROM EmployeeHierarchy;

Output

id | name    | manager_id
---|---------|-----------
1  | Alice   | NULL
2  | Bob     | 1
3  | Charlie | 1
4  | Diana   | 2

When to Use

Use recursive CTEs when you need to work with hierarchical or connected data, such as organizational charts, folder structures, or bill of materials. They are perfect for queries where you want to find all related items starting from one point and moving through levels.

For example, you can find all subordinates under a manager, all parts needed to build a product, or all comments in a threaded discussion. Recursive CTEs simplify these tasks by avoiding complicated loops in application code.

Key Points

• A recursive CTE has two parts: an anchor query and a recursive query.
• The anchor query runs once to get the starting rows.
• The recursive query runs repeatedly, joining new rows to previous results.
• It stops when no new rows are added.
• Recursive CTEs make hierarchical queries easier and more readable.

Key Takeaways

Recursive CTEs let you query hierarchical data by repeatedly joining rows until no more are found.

They consist of an anchor part and a recursive part that calls itself.

Use recursive CTEs for organizational charts, folder trees, and other connected data.

MySQL 8 introduced support for recursive CTEs, making complex queries simpler.

Recursive CTEs improve readability and reduce the need for procedural code.