Recursive CTE is a tool for processing hierarchical data in SQL, suitable for organizational structures, classification trees and other scenarios. It consists of anchor members and recursive members, the former defines the initial query, and the latter connects the next level of data through the JOIN operation. For example, when looking for all subordinates under a manager in an employee table, use WITH RECURSIVE to start with and refer to itself to implement recursive queries. In actual applications, performance issues, infinite loop risks and database compatibility differences should be paid attention to. Common uses include displaying organizational structures, obtaining subcategories, building comment trees, etc. 1. The anchor member defines the root node; 2. Recursive member connects the previous layer of results; 3. To avoid infinite loops, you need to set hierarchical restrictions; 4. The database support is slightly different; 5. It is often used in tree structure query.
Recursive querying becomes a very practical tool when you need to process hierarchical data, such as organizational structures, classification trees, or comment chains. In SQL, using CTE (Common Table Expression) to implement recursive query is one of the most common methods.
Here are some points you may be concerned about and practical suggestions on how to implement recursive queries with CTE.
What is recursive CTE?
Recursive CTE is a type of CTE that can reference its own within itself, which is usually used to query data with parent-child relationships. It consists of two parts:
- Anchor Member : Defines the initial query, usually the root node or the first layer of data.
- Recursive Member : Connect the results of the previous layer to the next layer through a JOIN operation.
The end result is a collection of all levels of data.
For example, if you have an employee table where each employee has a manager_id pointing to its superior, you can use a recursive CTE to find all the subordinates under a manager.
How to write a basic recursive CTE query?
Taking PostgreSQL as an example, the syntax is roughly as follows:
WITH RECURSIVE cte_name AS (
-- Anchor member SELECT ...
FROM ...
WHERE ...
UNION ALL
-- Recursive member SELECT...
FROM ...
INNER JOIN cte_name ON ...
)
SELECT * FROM cte_name;Key points:
- Start with
WITH RECURSIVE. - There must be at least one non-recursive query as the starting point (anchor member).
- The recursive part must refer to itself (cte_name).
- To avoid infinite loops, you can set maximum depth limits (such as
WHERE level < 100).
For example, find all subordinate employees starting with the CEO:
WITH RECURSIVE org_chart AS (
SELECT employee_id, name, manager_id, 1 AS level
FROM employees
WHERE manager_id IS NULL -- Assume that the CEO's manager_id is NULL
UNION ALL
SELECT e.employee_id, e.name, e.manager_id, oc.level 1
FROM employees e
INNER JOIN org_chart oc ON e.manager_id = oc.employee_id
)
SELECT * FROM org_chart;Issues that need to be paid attention to in practical applications
In actual use, there are several error-prone places:
- Performance issues : Recursive queries may affect performance if they involve a large amount of data. Consider adding indexes, especially on join fields (such as
manager_id). - Infinite loop risk : If there is a circular reference in the data (for example, A is the superior of B, and B is the superior of A), infinite recursion will be caused. It can be prevented by adding a level counter (such as the
levelfield above) and setting an upper limit. - Database compatibility : Different databases support for recursive CTEs is slightly different. For example, MySQL 8.0 supports it, but Oracle uses
START WITH ... CONNECT BYsyntax, and SQL Server is similar to PostgreSQL.
What are the common uses?
Recursive CTE is very useful in many scenarios, such as:
- Show the entire organizational chart
- Get the category directory and its subcategories
- Build a comment reply tree structure
- Find paths, network topology, etc.
For example, an e-commerce platform can use it to obtain all subcategories under a main category to display the complete product category.
Overall, recursive CTE is a powerful way to process tree-like structured data. Although the syntax seems a bit complicated, as long as you master the basic structure, you can flexibly respond to various hierarchical query needs. Basically that's it.
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