Recursive CTE is a query function introduced by MySQL 8.0 for processing hierarchical structure data. 1. It consists of non-recursive initial query and recursive parts, and traverses the tree structure by repeatedly referring to its own results; 2. When building, you need to clarify the initial conditions and recursive logic, and pay attention to avoiding circular references, adding hierarchical fields, and optimizing indexes; 3. Common application scenarios include organizational structure display, classification directory traversal, comment nested display and path search; 4. When using, you need to pay attention to controlling the depth of recursiveness to prevent infinite loops, optimize performance, reasonable sorting, and ensure version compatibility.

MySQL's recursive query function is implemented through recursive common table expressions (CTEs), which are mainly used to process data with hierarchical structures, such as organizational structures, classification trees, comment nesting, etc. In MySQL 8.0, recursive CTE was officially introduced, greatly enhancing the ability to query hierarchical data.

What is recursive CTE?

Recursive CTE is a query structure that can refer to its own results multiple times. It consists of one or more non-recursive members and one recursive member, which is usually used to traverse a tree structure. The basic structure is as follows:

 WITH RECURSIVE cte_name AS (
    -- Non-recursive part (initial query)
    SELECT ...
    UNION ALL
    -- Recursive part SELECT ...
    FROM cte_name
)
SELECT * FROM cte_name;

The non-recursive part is responsible for providing the initial data, while the recursive part constantly generates new data from existing results until no new records are generated.

How to build a recursive CTE query?

The key to building a recursive CTE is to understand the data structure and control the depth of recursiveness. Here is a typical example, suppose there is an employee table employees where each employee has a manager_id field pointing to its superior:

 WITH RECURSIVE employee_hierarchy AS (
    -- Initial query: Select CEO, that is, employees without superiors SELECT employee_id, name, manager_id, 1 AS level
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    -- Recursive query: Find the subordinates of each employee SELECT e.employee_id, e.name, e.manager_id, eh.level 1
    FROM employees e
    INNER JOIN employee_hierarchy eh
        ON e.manager_id = eh.employee_id
)
SELECT * FROM employee_hierarchy;

This query will start from the CEO, expand the entire organizational structure layer by layer, and record each person's "level" in the level.

Pay attention to when building:

• Recursive termination condition : Ensure that recursion can end naturally and avoid infinite loops. For example, employees will not have circular references (A is B's superior, and B is A's superior).
• Field selection : It is recommended to add hierarchical fields during recursion to facilitate subsequent analysis.
• Performance optimization : Recursive queries may involve a large amount of data, and proper use of indexes (such as manager_id fields) can significantly improve efficiency.

What are the common application scenarios?

Recursive CTE is suitable for scenarios where hierarchies are required. Here are a few common uses:

• Organizational structure display : a complete hierarchical structure from CEO to ordinary employees.
• Category directory traversal : For example, the product classification tree of e-commerce platforms.
• Comments nesting display : the hierarchical relationship between comments and replies in a blog or forum.
• Path search : Find the complete path from a certain node to the root node in the graph structure.

For example, in a forum system, using recursive CTE can easily find all subcomments of a comment:

 WITH RECURSIVE comment_tree AS (
    SELECT comment_id, parent_id, content, 1 AS depth
    FROM comments
    WHERE parent_id IS NULL -- Initial Comment (root Comment)

    UNION ALL

    SELECT c.comment_id, c.parent_id, c.content, ct.depth 1
    FROM comments c
    JOIN comment_tree ct ON c.parent_id = ct.comment_id
)
SELECT * FROM comment_tree;

This allows you to obtain a complete comment tree structure at one time, making it convenient for front-end rendering.

What issues should be paid attention to when using recursive CTE?

Although recursive CTE is powerful, there are some things to pay attention to during use:

• Avoid infinite recursion : If there is a circular reference in the data, it will cause the query to fail to terminate. System crashes can be prevented by setting the maximum recursion depth ( cte_max_recursion_depth ).

• Performance issues : Recursive queries may put a lot of pressure on the database, especially when the data volume is large or the level is deep. The index should be optimized as much as possible and unnecessary field returns should be avoided.

• Result sorting : The results of recursive CTE are unordered by default. If you need to display them in hierarchy, you can add sorting fields in the query, such as ORDER BY level .

• Version compatibility : Recursive CTE is only supported on MySQL 8.0 and above, and cannot be used in older versions.

If you encounter scenarios that need to deal with tree structures in development, recursive CTE is a very practical tool. As long as you pay attention to structural design and performance control, complex hierarchical query can be completed efficiently.

Basically that's it.

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