create table t_emp(
	id int primary key, 
	name varchar(20),
	deptId int
);
create table t_dept(
	id int primary key,
	name varchar(20)
);
insert into t_dept(id, name) values(1, '設計部');
insert into t_dept(id, name) values(2, '開發部');
insert into t_dept(id, name) values(3, '測試部');
insert into t_emp(id, name, deptId) values(1, '張三', 1);
insert into t_emp(id, name, deptId) values(2, '李四', 2);
insert into t_emp(id, name, deptId) values(3, '王五', 0);
# ps：為了說明方便，t_emp 表 說成 A 表， t_dept 表說成 B 表

SELECT * FROM t_emp e INNER JOIN t_dept d ON  e.deptId = d.id;

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.deptId = d.id;

SELECT * FROM t_emp e RIGHT JOIN t_dept d ON e.deptId = d.id;

SELECT * FROM t_emp e LEFT JOIN t_dept d 
ON e.deptId = d.id UNION 
SELECT * FROM t_emp e RIGHT JOIN t_dept d ON e.deptId = d.id;

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.deptId= d.id WHERE d.id is null;

SELECT * FROM t_emp e RIGHT JOIN t_dept d ON e.deptId= d.id WHERE e.id is null;

SELECT * FROM t_emp e LEFT JOIN t_dept d ON e.deptId= d.id WHERE d.id is null
UNION
SELECT * FROM t_emp e RIGHT JOIN t_dept d ON e.deptId= d.id WHERE e.id is null;