SQL Tutorial: Understanding Recursive CTE in SQL.

Understanding Recursive CTE in SQL

In SQL, a Common Table Expression (CTE) is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. It allows you to create a named temporary result set that can be used multiple times within a query.

A recursive CTE, also known as a recursive common table expression, is a special type of CTE that allows you to perform recursive queries. It is particularly useful when dealing with hierarchical or recursive data structures, such as organizational charts, file systems, or bill of materials.

How Recursive CTE Works

A recursive CTE consists of two parts: the anchor member and the recursive member.

The anchor member is the initial query that forms the base case for the recursion. It is the starting point of the recursive query. The anchor member is executed once at the beginning of the query execution.

The recursive member is the query that references the CTE itself. It defines how the result set is built recursively. The recursive member is executed repeatedly until the termination condition is met.

Each iteration of the recursive member takes the previous iteration’s result set and applies additional filters or joins to generate the next iteration’s result set. This process continues until the termination condition is met, or until the maximum recursion level is reached.

Example of Recursive CTE

Let’s consider an example to understand how recursive CTE works. Suppose we have a table called “Employees” with the following structure:

EmployeeID | EmployeeName | ManagerID
-----------|--------------|----------
1          | John         | NULL
2          | Mary         | 1
3          | David        | 2
4          | Sarah        | 2
5          | Michael      | 1

We want to retrieve all the employees in the organization hierarchy, starting from the top-level manager. We can use a recursive CTE to achieve this:

WITH RecursiveCTE AS (
    -- Anchor member
    SELECT EmployeeID, EmployeeName, ManagerID
    FROM Employees
    WHERE ManagerID IS NULL
    
    UNION ALL
    
    -- Recursive member
    SELECT e.EmployeeID, e.EmployeeName, e.ManagerID
    FROM Employees e
    INNER JOIN RecursiveCTE r ON e.ManagerID = r.EmployeeID
)
SELECT *
FROM RecursiveCTE;

In this example, the anchor member selects all employees who do not have a manager (i.e., top-level managers). The recursive member then joins the Employees table with the RecursiveCTE to retrieve the employees who report to each manager.

The UNION ALL operator combines the anchor member and the recursive member’s result sets. The result set is then used as the input for the next iteration of the recursive member. This process continues until all levels of the hierarchy are retrieved.

The final SELECT statement outside the CTE retrieves the result set from the RecursiveCTE, which contains all the employees in the organization hierarchy.

Conclusion

Recursive CTEs are a powerful feature in SQL that allows you to work with hierarchical or recursive data structures. By using the anchor member and the recursive member, you can build complex queries that retrieve data in a recursive manner.

In this blog post, we discussed the concept of recursive CTEs and provided an example to demonstrate how they work. By understanding how recursive CTEs function, you can leverage this feature to solve various problems in SQL.