with recursive

With recursive is a clause used in SQL queries to perform recursive operations on hierarchical data structures. It allows you to repeatedly query a table and its related tables, building up a result set by combining the results of each iteration.

By using the WITH RECURSIVE clause, you can define a recursive query that consists of two parts: the base case and the recursive case. The base case represents the starting point of the recursion, while the recursive case defines how to transform the result set from the previous iteration into the result set for the current iteration.

Here's an example to illustrate how it works:

WITH RECURSIVE RecursiveCTE (id, name, parent_id, level) AS (
  SELECT id, name, parent_id, 0 
  FROM employees 
  WHERE name = 'John'
  
  UNION ALL
  
  SELECT e.id, e.name, e.parent_id, rc.level + 1
  FROM employees e
  INNER JOIN RecursiveCTE rc ON e.parent_id = rc.id
)
SELECT id, name, level
FROM RecursiveCTE;

In this example, we have an "employees" table with columns like "id", "name", and "parent_id". We want to find all the employees that are in the hierarchical chain above "John", including "John" himself. The recursive query starts with the base case where we select "John" and set the initial level to 0. Then, in the recursive case, we join the "employees" table with the RecursiveCTE (the previous iteration of the query) to find employees who have their parent_id equal to the id of the previous iteration. The recursion continues until no more matches are found.

The final result of the query is a result set that includes all the employees in the hierarchical chain above "John", along with their respective levels in the hierarchy.

Note that the specific syntax and capabilities of the WITH RECURSIVE clause may vary depending on the database management system you are using.