mysql的做外链接查询

-- employees 表
CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    department_id INT
);

-- departments 表
CREATE TABLE departments (
    id INT PRIMARY KEY,
    name VARCHAR(100)
);

-- 插入一些示例数据
INSERT INTO employees (id, name, department_id) VALUES
(1, 'Alice', 1),
(2, 'Bob', 2),
(3, 'Charlie', NULL);

INSERT INTO departments (id, name) VALUES
(1, 'HR'),
(2, 'Engineering');

SELECT e.name AS employee_name, d.name AS department_name
FROM employees e
LEFT OUTER JOIN departments d ON e.department_id = d.id;

SELECT e.name AS employee_name, d.name AS department_name
FROM employees e
RIGHT OUTER JOIN departments d ON e.department_id = d.id;

SELECT e.name AS employee_name, COALESCE(d.name, 'No Department') AS department_name
FROM employees e
LEFT OUTER JOIN departments d ON e.department_id = d.id;

SELECT e.name AS employee_name, d.name AS department_name
FROM employees e
LEFT OUTER JOIN departments d ON e.department_id = d.id
UNION ALL
SELECT e.name AS employee_name, d.name AS department_name
FROM employees e
RIGHT OUTER JOIN departments d ON e.department_id = d.id
WHERE e.id IS NULL;