Write Recursive Queries in MySQL

In this tutorial, we'll how to write Recursive Queries in MySQL.

Recursive queries in MySQL are valuable for handling hierarchical or tree-structured data, such as organizational structures, category hierarchies, or dependency chains. In MySQL, recursive queries are achieved using Common Table Expressions (CTEs), introduced in MySQL 8.0. In this guide, we’ll walk through the process of writing recursive queries, explaining each step and demonstrating how you can apply these concepts to real-world examples.

Prerequisites

• MySQL Server installed on your dedicated server or KVM VPS
• MySQL 8.0 or Higher: Recursive CTEs are available starting from MySQL 8.0, so ensure you have this version or newer.
• Basic SQL Knowledge: Understanding basic SQL concepts, especially CTEs, will make it easier to follow along.

1. Understanding Recursive CTEs in MySQL

A recursive query allows a query to refer to itself using a CTE, which is a temporary result set. Recursive CTEs are typically used for hierarchical or tree-structured data. A recursive CTE in MySQL has two parts:

• Anchor Member: The initial query that starts the recursion.
• Recursive Member: The part of the query that references the CTE, allowing the query to call itself.

Here’s the basic syntax for a recursive CTE:

WITH RECURSIVE cte_name (column1, column2, ...) AS (
    anchor_query
    UNION ALL
    recursive_query
)
SELECT * FROM cte_name;

2. Creating a Sample Table and Data

Let's consider an example of an employee hierarchy where each employee has a manager. We'll create a table called employees:

CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    employee_name VARCHAR(100),
    manager_id INT
);

Next, insert some sample data to simulate a hierarchical structure:

INSERT INTO employees (employee_id, employee_name, manager_id) VALUES
(1, 'Alice', NULL),      -- Alice is the CEO, with no manager
(2, 'Bob', 1),           -- Bob reports to Alice
(3, 'Charlie', 1),       -- Charlie reports to Alice
(4, 'David', 2),         -- David reports to Bob
(5, 'Eve', 2),           -- Eve reports to Bob
(6, 'Frank', 3);         -- Frank reports to Charlie

3. Writing Recursive Queries

Objective: Retrieve all employees under a specific manager, showing the hierarchy levels.

The recursive CTE will allow us to build a query to find all employees under a given manager. In this example, we’ll find all employees under Alice (employee_id = 1).

Here's how to write the recursive query:

WITH RECURSIVE employee_hierarchy AS (
    -- Anchor Member: Select Alice's direct reports
    SELECT employee_id, employee_name, manager_id, 1 AS level
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    -- Recursive Member: Find employees reporting to each level
    SELECT e.employee_id, e.employee_name, e.manager_id, eh.level + 1
    FROM employees AS e
    INNER JOIN employee_hierarchy AS eh ON e.manager_id = eh.employee_id
)
SELECT * FROM employee_hierarchy;

Explanation of the query:

• Anchor Member: Starts with employees who don’t have a manager (e.g., the top level in the hierarchy).
• Recursive Member: Finds employees whose manager_id matches an employee_id in the current level, incrementing the level by 1.

4. Examples of Recursive Queries

Example 1: Retrieve Hierarchical Employee Tree Starting from a Specific Manager

If you want to retrieve a tree for a particular manager (say, Bob), you can modify the anchor query to start with Bob:

WITH RECURSIVE employee_hierarchy AS (
    SELECT employee_id, employee_name, manager_id, 1 AS level
    FROM employees
    WHERE employee_id = 2  -- Bob's employee_id

    UNION ALL

    SELECT e.employee_id, e.employee_name, e.manager_id, eh.level + 1
    FROM employees AS e
    INNER JOIN employee_hierarchy AS eh ON e.manager_id = eh.employee_id
)
SELECT * FROM employee_hierarchy;

Example 2: Calculating the Depth of a Hierarchical Structure

This example calculates the depth of the hierarchy from the top level downwards:

WITH RECURSIVE hierarchy_depth AS (
    SELECT employee_id, employee_name, manager_id, 1 AS depth
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    SELECT e.employee_id, e.employee_name, e.manager_id, hd.depth + 1
    FROM employees AS e
    INNER JOIN hierarchy_depth AS hd ON e.manager_id = hd.employee_id
)
SELECT MAX(depth) AS max_hierarchy_depth FROM hierarchy_depth;

5. Best Practices for Recursive Queries

• Limit Depth When Possible: Recursive queries can run indefinitely if there’s a circular reference. Limit the depth to avoid infinite loops. For instance, you could add a condition like WHERE level <= 10 to stop the recursion.
• Use UNION ALL for Performance: Using UNION instead of UNION ALL will remove duplicates but can slow down performance. Prefer UNION ALL unless duplicates need to be removed.
• Check for Circular References: Ensure your data does not contain circular references. You can add constraints to the table to prevent circular references or handle them in your query.

6. Limitations and Performance Considerations

Resource Intensive: Recursive CTEs can be resource-intensive, especially with large datasets. Limit recursion depth or optimize queries to manage resources efficiently.
Limited Support for Complex Hierarchies: While recursive queries work well for simple hierarchies, more complex structures might require additional query optimization or even rethinking the database schema.
Alternative Approaches: In some cases, nested set models or materialized path models can perform better than recursive queries for certain applications.

Conclusion

Recursive queries in MySQL are a powerful feature for working with hierarchical data. Using CTEs, you can query parent-child relationships effectively, making it easier to handle organizational structures or any tree-like data. By understanding and implementing recursive CTEs, you can build robust queries to solve complex data requirements in MySQL.

Recursive queries should always be approached with best practices and limitations in mind to ensure optimal performance and scalability.