SQL Oracle LEFT JOIN和SUBQUERY错误: ORA-00905:缺少关键字

CREATE TABLE employees (
    employee_id NUMBER,
    first_name VARCHAR2(50),
    last_name VARCHAR2(50)
);

CREATE TABLE departments (
    department_id NUMBER,
    department_name VARCHAR2(50)
);

SELECT e.employee_id, e.first_name, e.last_name, d.department_name
FROM employees e
LEFT JOIN departments d ON e.department_id = d.department_id;

SELECT e.employee_id, e.first_name, e.last_name, (
    SELECT d.department_name
    FROM departments d
    WHERE d.department_id = e.department_id
) AS department_name
FROM employees e
LEFT JOIN departments d ON e.department_id = d.department_id;

SELECT e.employee_id, e.first_name, e.last_name, d.department_name
FROM employees e
LEFT JOIN (
    SELECT department_id, department_name
    FROM departments
) d ON e.department_id = d.department_id;