join

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;