PostgreSQL: Unexpected Behavior with CTEs and Recursive Queries
Hey everyone, I'm running into an issue that's driving me crazy. I'm maintaining legacy code that I'm dealing with I'm reviewing some code and I'm experiencing unexpected behavior when using Common Table Expressions (CTEs) in PostgreSQL 13.3, specifically with recursive queries. I have a table `employees` structured as follows: ```sql CREATE TABLE employees ( id SERIAL PRIMARY KEY, name VARCHAR(100), manager_id INT REFERENCES employees(id) ); ``` I want to retrieve a list of all employees along with their hierarchical levels based on their managers. I thought I could achieve this with a recursive CTE, but I'm facing issues where the query returns duplicate rows for some employees. Here's my query: ```sql WITH RECURSIVE employee_hierarchy AS ( SELECT id, name, manager_id, 0 AS level FROM employees WHERE manager_id IS NULL UNION ALL SELECT e.id, e.name, e.manager_id, eh.level + 1 FROM employees e JOIN employee_hierarchy eh ON e.manager_id = eh.id ) SELECT * FROM employee_hierarchy; ``` When I run this, I get duplicates for some employees that have multiple managers, which seems incorrect to me. For instance, if an employee has two different entries in the `employees` table with the same `name` but different `id`s and linked manager IDs, I see duplicate names in the result with different levels. I've tried adding `DISTINCT` in the final selection: ```sql SELECT DISTINCT * FROM employee_hierarchy; ``` But that doesn't solve the issue because it still doesn't account for how levels are being calculated. I also checked if there were any circular references in the data, but that doesn't seem to be the case. Does anyone have insights on how to properly structure this recursive CTE to avoid duplicates and correctly represent the hierarchy? Any best practices or common pitfalls I might be missing would be greatly appreciated! I've been using Sql for about a year now. Thanks for taking the time to read this! I'm developing on CentOS with Sql.