left join과 right join을 union하면 됩니다.
아래 블로그에 좋은 그림이 있네요.
http://blog.naver.com/PostView.nhn?blogId=parkjy76&logNo=220784188333&parentCategoryNo=&categoryNo=14&viewDate=&isShowPopularPosts=true&from=search
SELECT * FROM doctor LEFT JOIN employee ON (doctor.ID = employee.ID)
UNION
SELECT * FROM doctor RIGHT JOIN employee ON (doctor.ID = employee.ID);