SQL学习笔记九种连接2

等值连接 vs 非等值连接

SELECT last_name,salary ,job_grades.grade_level from employees,job_grades
WHERE employees.salary >= job_grades.lowest_sal AND employees.salary < job_grades.highest_sal;

外连接，内连接

左外连接，右外连接
-sql 92语法实现外连接
SELECT a.first_name,a.employee_id,a.department_id FROM employees a ,departments b
WHERE a.department_id = b.department_id(+);
-mysql不支持sql92语法-

-99语法左外连接

SELECT a.first_name,a.employee_id,a.department_id FROM employees a LEFT OUTER JOIN departments b
ON a.department_id = b.department_id;
outer可以省略掉
左外连接查出来的是左边的一整张图
右外连接是右边一整张图

union

由于sql中full join不能用到，因此使用union取两个表的并集

• union all 两个图a图和b图进行合并，重复部分多余有一份，虽然冗余，但是开发中尽量用union all

中图：内连接
SELECT a.first_name,a.employee_id,a.department_id FROM employees a JOIN departments b
ON a.department_id = b.department_id;

左上图
SELECT a.first_name,a.employee_id,a.department_id FROM employees a LEFT JOIN departments b
ON a.department_id = b.department_id;

右上图
SELECT a.first_name,a.employee_id,a.department_id FROM employees a RIGHT JOIN departments b
ON a.department_id = b.department_id;

左中图

SELECT a.first_name,a.employee_id,a.department_id FROM employees a LEFT JOIN departments b
ON a.department_id = b.department_id
WHERE a.department_id = NULL;

右中图

SELECT a.first_name,a.employee_id,a.department_id FROM employees a RIGHT JOIN departments b
ON a.department_id = b.department_id
WHERE b.deparment_id = NULL;

满外连接
左上图 UNION ALL 右中图
SELECT a.first_name,a.employee_id,a.department_id FROM employees a LEFT JOIN departments b
ON a.department_id = b.department_id;
UNION ALL
SELECT a.first_name,a.employee_id,a.department_id FROM employees a RIGHT JOIN departments b
ON a.department_id = b.department_id
WHERE b.deparment_id = NULL;