Recursive queries in SQL are primarily handled using Common Table Expressions (CTEs). A recursive CTE allows you to query hierarchical or tree-structured data, enabling you to retrieve results based on relationships in the data. This means you can effectively find all items in a nested structure, such as organizational charts or product categories, where records reference themselves. In a recursive CTE, there are two main components: the anchor member and the recursive member.
The anchor member is the initial query that selects the starting point of the recursion. This might be a specific node in a hierarchy, such as a manager in an employee table. The recursive member then builds upon the results of the anchor member and executes repeatedly until no new rows are returned. For example, if you're looking to list all employees under a specific manager, the anchor might select the manager, and the recursive part would retrieve all direct reports of that manager, then their reports, and so on, effectively traversing the hierarchy.
Here's an example of a recursive CTE in action. Imagine you have an employees table with columns id, name, and manager_id to indicate who reports to whom. You can create a recursive query like this:
WITH RECURSIVE EmployeeHierarchy AS (
SELECT id, name, manager_id
FROM employees
WHERE manager_id IS NULL -- starting with top-level managers
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;
In this example, the CTE first selects top-level employees with no manager, then it recursively joins the employees table to find all employees reporting to each manager, allowing you to view the full hierarchy efficiently.