注意:这些不是家庭作业问题。我正在学习dbms,因此这些作业类似的问题。高效SQL查询
两个表:
Teachers (teacher_id, teacher_name)
Courses (teacher_id,course_id, course_name)
为了选择谁不教任何课程的老师的名字,有 两个查询我能想到的:
mysql> explain select teacher_name from teachers left join courses on (teachers.
teacher_id = courses.teacher_id) where course_id is null;
mysql> explain select teacher_name from teachers where teacher_id not in (select
teacher_id from courses);
哪一个会更有效?为什么?
您需要将TeacherID添加到课程表中。 – 2009-04-19 04:19:23