连接两个表sqlplus，共享列名，返回NULL

-- 创建示例表
CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    name VARCHAR(100),
    department_id INT
);

CREATE TABLE departments (
    department_id INT PRIMARY KEY,
    department_name VARCHAR(100)
);

-- 插入示例数据
INSERT INTO employees (employee_id, name, department_id) VALUES (1, 'Alice', 1);
INSERT INTO employees (employee_id, name, department_id) VALUES (2, 'Bob', 2);
INSERT INTO departments (department_id, department_name) VALUES (1, 'HR');
INSERT INTO departments (department_id, department_name) VALUES (2, 'Engineering');

-- 使用表别名进行连接查询
SELECT e.employee_id, e.name, e.department_id AS emp_dept_id, d.department_name, d.department_id AS dept_dept_id
FROM employees e
JOIN departments d ON e.department_id = d.department_id;

SELECT employee_id, name, department_id, department_name
FROM employees
JOIN departments ON employees.department_id = departments.department_id;