7种JOIN的实现：
中图：内连接
SELECT employee_id,department_name
FROM employees e JOIN departments d
ON e.department_id = d.department_id;
左上图：左外连接
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id;
右上图：右外连接
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id;
左中图：
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL;
右中图：
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL;
左下图：满外连接
方式1：左上图 UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL;
方式2：左中图 UNION ALL 右上图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id;
右下图：左中图 UNION ALL 右中图
SELECT employee_id,department_name
FROM employees e LEFT JOIN departments d
ON e.department_id = d.department_id
WHERE d.department_id IS NULL
UNION ALL
SELECT employee_id,department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL;
