Can you explain what recursive expressions of common table expressions (CTE) with complex conditions are?

Recursive expressions of common table expressions (CTE) are a special type of CTE in SQL that allows executing recursive queries, for example, to traverse hierarchies or graphs.

Typically, a recursive CTE consists of two parts:

1. Base query — the initial set of rows.
2. Recursive query — references itself, adding new rows at each iteration.

"Complex conditions" may mean that additional filters, joins, or calculations are used in the recursive part, complicating the recursion logic.

An example of a recursive CTE for traversing an employee hierarchy:

WITH RECURSIVE EmployeeHierarchy AS (
    -- base query: select the manager
    SELECT id, manager_id, name, 1 AS level
    FROM employees
    WHERE manager_id IS NULL

    UNION ALL

    -- recursive query: select subordinates
    SELECT e.id, e.manager_id, e.name, eh.level + 1
    FROM employees e
    INNER JOIN EmployeeHierarchy eh ON e.manager_id = eh.id
    WHERE e.status = 'active' -- example of a complex condition
)
SELECT * FROM EmployeeHierarchy;

Here, WHERE e.status = 'active' is an additional condition that complicates the recursion.

Thus, recursive CTEs enable efficient work with hierarchical data, and complex conditions help filter or modify results at each recursion step.