¿Cuáles son los pasos para implementar consultas recursivas en SQL?

CTE queries are a got to for making representations of hierarchical data. For example an org structure. The key for formulating is to anchor against the top record - so in this example the CEO, then resolve the reports to the CEO, etc as part of a top down traverse. It is important to set a recursion limit. It is useful to increment a level counter so the resulting query shows the recursion depth.

Create Recursive CTE: Use WITH RECURSIVE to define a recursive common table expression (CTE). Base Query: Write a base query selecting the initial set of data with a specified condition (e.g., where parent_id is NULL). Recursive Query: In the recursive part, join the table with the recursive CTE to fetch related rows. Union Base and Recursive Queries: Combine the base and recursive queries using UNION ALL within the CTE. Select from Recursive CTE: Use a SELECT statement outside the CTE to retrieve the final result set. For optimizing the performance of recursive queries in SQL, tips are: Indexes, Analyze Execution Plan, Limit Result Set, Avoid Excessive Recursion, Use Proper Data Types, Partitioning (if applicable), and Batch Processing.

A simple example of a company structure can be written like this in SQL Server: declare @emp table (EmployeeID INT PRIMARY KEY,EmployeeName NVARCHAR(50), ManagerID INT) INSERT INTO @emp (EmployeeID, EmployeeName, ManagerID) VALUES (1, 'John', NULL),(2, 'Alice', 1),(3, 'Bob', 1),(4, 'Charlie', 2),(5, 'David', 2),(6, 'Eva', 3),(7, 'Frank', 3); WITH EmployeeHierarchyCTE AS ( SELECT EmployeeID,EmployeeName,ManagerID, 0 AS Level FROM @emp WHERE ManagerID IS NULL UNION ALL SELECT e.EmployeeID, e.EmployeeName, e.ManagerID, eh.Level + 1 AS Level FROM @emp e INNER JOIN EmployeeHierarchyCTE eh ON e.ManagerID = eh.EmployeeID ) SELECT * FROM EmployeeHierarchyCTE; Note the anchor to head (managerID is null)

To set the depth of recursion in SQL Server there is the command OPTION (MAXRECURSION <number>) To enable bottomless set to 0

Ensure that columns used in WHERE clauses are appropriately indexed to speed up the filtering process. And indexing columns which are involved in the JOIN conditions can significantly improve the performance of recursive queries.

To optimise performance ensure there is an index on the on the join fields/foreign key. Also set the recursion depth to prevent it blowing out.

Recursive query would be a very powerful tool for manipulating data where straightforward aggregation functions (e.g. sum, average, max) can't solve the need. Typical examples would be to find the hierarchy information in the same table (like the employee case shown in this article), or splitting the data records based on certain conditions (e.g. the continuous phone usage splitting into blocks of 15 minutes of the same user). Mastering this skill will help data analysts handle more complex data analysis scenarios.