问题
单项选择题
在同一螺线上相邻两牙对应两点间的轴向距离称为（）。
A、螺距
B、导程
C、线数
D、旋向
答案
参考答案：B
搜题请搜索小程序“爱下问搜题”
在同一螺线上相邻两牙对应两点间的轴向距离称为（）。
A、螺距
B、导程
C、线数
D、旋向
参考答案：B
以下是CLASS(课程)表和INSTRUCTOR(教师)表中所存储的数据：
| CLASS | |||
| CLASS_ID | CLASS_NAME | HOURS_CREDIT | INSTRUCTOR_ID |
| 1 | Introduction to Accounting | 3 | 4 |
| 2 | Computer Basics | 3 | 1 |
| 3 | Tax Accounting Principles | 3 | 4 |
| 4 | American History | 3 | 2 |
| 5 | Basic Engineering | 3 | |
| INSTRUCTOR | ||
| INSTRUCTOR_ID | LAST_NAME | FIRST_NAME |
| 1 | Chao | Ling |
| 2 | Vanderbilt | Herbert |
| 3 | Wigley | Martha |
| 4 | Page | Albert |
A．DELETE class_id, class_name, hours_credit, instructor_id
FROM class
WHERE instructor_id IS NULL;
B．DELETE FROM class
WHERE instructor_id NOT IN
(SELECT instructor_id
FROM class);
C．DELETE FROM
instructor NATURAL JOIN class
WHERE instructor_id IS NOT NULL;
D．DELETE FROM class
WHERE instructor_id IS NULL;