mysql if循环查询层级

IF(condition, value_if_true, value_if_false)

SELECT 
    e1.employee_name,
    IF(e1.manager_id IS NOT NULL, e2.employee_name, 'No Manager') AS manager_name
FROM 
    employees e1
LEFT JOIN 
    employees e2 ON e1.manager_id = e2.employee_id;

WITH RECURSIVE employee_hierarchy AS (
    SELECT 
        employee_id, 
        employee_name, 
        manager_id, 
        1 AS level
    FROM 
        employees
    WHERE 
        manager_id IS NULL
    UNION ALL
    SELECT 
        e.employee_id, 
        e.employee_name, 
        e.manager_id, 
        eh.level + 1
    FROM 
        employees e
    INNER JOIN 
        employee_hierarchy eh ON e.manager_id = eh.employee_id
)
SELECT * FROM employee_hierarchy;