一、表关系

请创建如下表，并创建相关约束

二、操作表

1、自行创建测试数据

/*

Navicat MySQL Data Transfer

Source Server         : mysql5.7.1

Source Server Version : 50717

Source Host           : localhost:3306

Source Database       : crm

Target Server Type    : MYSQL

Target Server Version : 50717

File Encoding         : 65001

Date: 2017-02-24 09:50:15

*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------

-- Table structure for class

-- ----------------------------

DROP TABLE IF EXISTS `class`;

CREATE TABLE `class` (

  `cid` int(11) NOT NULL AUTO_INCREMENT,

  `caption` varchar(32) NOT NULL,

  PRIMARY KEY (`cid`)

) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of class

-- ----------------------------

INSERT INTO `class` VALUES ('', '三年二班');

INSERT INTO `class` VALUES ('', '三年三班');

INSERT INTO `class` VALUES ('', '一年二班');

INSERT INTO `class` VALUES ('', '二年九班');

-- ----------------------------

-- Table structure for course

-- ----------------------------

DROP TABLE IF EXISTS `course`;

CREATE TABLE `course` (

  `cid` int(11) NOT NULL AUTO_INCREMENT,

  `cname` varchar(32) NOT NULL,

  `teacher_id` int(11) NOT NULL,

  PRIMARY KEY (`cid`),

  KEY `fk_course_teacher` (`teacher_id`),

  CONSTRAINT `fk_course_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`tid`)

) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of course

-- ----------------------------

INSERT INTO `course` VALUES ('', '生物', '');

INSERT INTO `course` VALUES ('', '物理', '');

INSERT INTO `course` VALUES ('', '体育', '');

INSERT INTO `course` VALUES ('', '美术', '');

-- ----------------------------

-- Table structure for score

-- ----------------------------

DROP TABLE IF EXISTS `score`;

CREATE TABLE `score` (

  `sid` int(11) NOT NULL AUTO_INCREMENT,

  `student_id` int(11) NOT NULL,

  `course_id` int(11) NOT NULL,

  `num` int(11) NOT NULL,

  PRIMARY KEY (`sid`),

  KEY `fk_score_student` (`student_id`),

  KEY `fk_score_course` (`course_id`),

  CONSTRAINT `fk_score_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`cid`),

  CONSTRAINT `fk_score_student` FOREIGN KEY (`student_id`) REFERENCES `student` (`sid`)

) ENGINE=InnoDB AUTO_INCREMENT=53 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of score

-- ----------------------------

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

INSERT INTO `score` VALUES ('', '', '', '');

-- ----------------------------

-- Table structure for student

-- ----------------------------

DROP TABLE IF EXISTS `student`;

CREATE TABLE `student` (

  `sid` int(11) NOT NULL AUTO_INCREMENT,

  `gender` char(1) NOT NULL,

  `class_id` int(11) NOT NULL,

  `sname` varchar(32) NOT NULL,

  PRIMARY KEY (`sid`),

  KEY `fk_class` (`class_id`),

  CONSTRAINT `fk_class` FOREIGN KEY (`class_id`) REFERENCES `class` (`cid`)

) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of student

-- ----------------------------

INSERT INTO `student` VALUES ('', '男', '', '刘洋');

INSERT INTO `student` VALUES ('', '女', '', '钢蛋');

INSERT INTO `student` VALUES ('', '男', '', '刘泽栋');

INSERT INTO `student` VALUES ('', '男', '', '张少博');

INSERT INTO `student` VALUES ('', '女', '', '张少强');

INSERT INTO `student` VALUES ('', '男', '', '王飞');

INSERT INTO `student` VALUES ('', '女', '', '铁锤');

INSERT INTO `student` VALUES ('', '男', '', '李三');

INSERT INTO `student` VALUES ('', '男', '', '李一');

INSERT INTO `student` VALUES ('', '女', '', '李二');

INSERT INTO `student` VALUES ('', '男', '', '李四');

INSERT INTO `student` VALUES ('', '女', '', '王鹏');

INSERT INTO `student` VALUES ('', '男', '', '刘明');

INSERT INTO `student` VALUES ('', '男', '', '刘刚');

INSERT INTO `student` VALUES ('', '女', '', '刘二');

INSERT INTO `student` VALUES ('', '男', '', '刘四');

-- ----------------------------

-- Table structure for teacher

-- ----------------------------

DROP TABLE IF EXISTS `teacher`;

CREATE TABLE `teacher` (

  `tid` int(11) NOT NULL AUTO_INCREMENT,

  `tname` varchar(32) NOT NULL,

  PRIMARY KEY (`tid`)

) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of teacher

-- ----------------------------

INSERT INTO `teacher` VALUES ('', '马云老师');

INSERT INTO `teacher` VALUES ('', '刘强东老师');

INSERT INTO `teacher` VALUES ('', '马化腾老师');

INSERT INTO `teacher` VALUES ('', '丁磊老师');

INSERT INTO `teacher` VALUES ('', '王建宁老师');

INSERT INTO `teacher` VALUES ('', '李杰老师');

2、操作

--

-- 1、自行创建测试数据

--

-- 2、查询“生物”课程比“物理”课程成绩高的所有学生的学号；

-- SELECT

-- 	A.student_id,A.num

-- FROM

-- 	(

-- 		SELECT

-- 			score.sid,

-- 			score.student_id,

-- 			course.cname,

-- 			score.num

-- 		FROM

-- 			score

-- 		LEFT JOIN course ON score.course_id = course.cid

-- 		WHERE

-- 			course.cname = "生物"

-- 	) AS A

--

-- INNER JOIN (

--

-- SELECT

-- 		score.sid,

-- 		score.student_id,

-- 		course.cname,

-- 		score.num

-- 	FROM

-- 		score

-- 	LEFT JOIN course ON score.course_id = course.cid

-- 	WHERE

-- 		course.cname = "物理"

-- ) AS B ON A.student_id = B.student_id

-- WHERE

-- 	A.num > B.num --

-- 3、查询平均成绩大于60分的同学的学号和平均成绩；

-- SELECT

-- 	B.student_id,

-- 	student.sname,

-- 	B.aa

-- FROM

-- 	(

-- 		SELECT

-- 			student_id,

-- 			avg(num) AS aa

-- 		FROM

-- 			score

-- 		GROUP BY

-- 			student_id

-- 		HAVING

-- 			avg(num) > 60

-- 	) AS B

-- LEFT JOIN student ON B.student_id = student.sid

-- 4、查询所有同学的学号、姓名、选课数、总成绩；

--  SELECT

-- 	score.student_id,

--  	count(course_id),

-- 	student.sname,

--  	sum(num)

--  FROM

--  	score

--  LEFT JOIN student ON score.student_id = student.sid

--  GROUP BY

--  	score.student_id;

--

--

-- 5、查询姓“李”的老师的个数；

-- select tid from teacher where tname like '李%';

--

-- 6、查询没学过“李平”老师课的同学的学号、姓名；

--  SELECT

-- 			 	student.sid,

-- 			 	student.sname

-- 			 FROM

-- 			 	student

-- 		 WHERE

-- 			sid NOT IN (

-- 				SELECT

-- 						student_id

-- 				FROM

-- 						score

-- 					WHERE

-- 		 			course_id IN (

-- 		 				SELECT

-- 			 					course.cid

-- 			 				FROM

-- 		 					course

-- 		 				LEFT JOIN teacher ON course.teacher_id = teacher.tid

-- 			 				WHERE

-- 								teacher.tname = "李平老师"

-- 			 			)

-- 			 		GROUP BY

-- 		 			student_id

-- 		 	)

--

--

-- 7、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；

-- SELECT

-- 	student_id,

-- 	student.sname

-- FROM

-- 	(

-- 		SELECT

-- 			student_id,

-- 			course_id

-- 		FROM

-- 			score

-- 		WHERE

-- 			score.course_id = 1

-- 		OR score.course_id = 2

-- 	) AS A

-- LEFT JOIN student ON A.student_id = student.sid

-- GROUP BY

-- 	student_id

-- HAVING

-- 	count(student_id) > 1;

-- 

-- 8、查询学过“李平”老师所教的所有课的同学的学号、姓名；

-- SELECT

-- 	student_id

-- FROM

-- 	score

-- WHERE

-- 	course_id IN (

-- 		SELECT

-- 			cid

-- 		FROM

-- 			course

-- 		LEFT JOIN teacher ON teacher_id = teacher.tid

-- 		WHERE

-- 			teacher.tname = "李平老师"

-- 	)

-- GROUP BY

-- 	student_id

-- HAVING

-- 	count(course_id) = (

-- 		SELECT

-- 			count(cid)

-- 		FROM

-- 			course

-- 		LEFT JOIN teacher ON teacher_id = teacher.tid

-- 		WHERE

-- 			teacher.tname = "李平老师"

-- 	)

-- 

-- 9、查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；

--  SELECT

--

-- A.student_id,A.num

--   FROM

--  	(

--  		SELECT

--  			score.sid,

--   			score.student_id,

-- 			score.course_id,

-- 			score.num

--  		FROM

-- 			score

--  		LEFT JOIN course ON score.course_id = course.cid

--  		WHERE

--  			score.course_id = 1

--   	) AS A

--

--   INNER JOIN

--

--  (

--  	SELECT

--   		score.sid,

-- 		score.student_id,

--  		score.course_id,

-- 		score.num

--   	FROM

--   		score

--   	LEFT JOIN course ON score.course_id = course.cid

--   	WHERE

--  		score.course_id = 2)

--

--   AS B ON A.student_id = B.student_id

--   WHERE

-- 	A.num > B.num

-- 

-- 10、查询有课程成绩小于60分的同学的学号、姓名；

--

-- SELECT

-- 	score.student_id,

-- 	student.sname

-- FROM

-- 	score

-- LEFT JOIN student ON score.student_id = student.sid

-- WHERE

-- 	score.num < 60

-- GROUP BY

-- 	student_id;

--

-- 11、查询没有学全所有课的同学的学号、姓名；

--   select

--  	student_id,

--   	student.sname

--   FROM

--   	score

--   LEFT JOIN student ON score.student_id = student.sid

--   GROUP BY

--  	score.student_id

--   HAVING count(1) < (select count(cid) from course);

--

--

-- 12、查询至少有一门课与学号为“001”的同学所学相同的同学的学号和姓名；

-- SELECT

-- 	student_id,

-- 	student.sname

-- FROM

-- 	score

-- LEFT JOIN student ON score.student_id = student.sid

-- WHERE

-- 	student_id != 1

-- AND course_id IN (

-- 	SELECT

-- 		course_id

-- 	FROM

-- 		score

-- 	WHERE

-- 		student_id = 1

-- )

-- GROUP BY

-- student_id

-- 13、查询至少学过学号为“001”同学所选课程中任意一门课的其他同学学号和姓名；

-- select course_id from score where  student_id=1;

-- SELECT

-- 	student_id,

-- 	count(1)

-- FROM

-- 	score

-- WHERE

-- 	student_id != 1

-- AND course_id IN (

-- 	SELECT

-- 		course_id

-- 	FROM

-- 		score

-- 	WHERE

-- 		student_id = 1

-- )

-- GROUP BY

-- 	student_id

-- HAVING

-- 	count(1) = (

-- 		SELECT

-- 			count(course_id)

-- 		FROM

-- 			score

-- 		WHERE

-- 			student_id = 1

-- 	)

-- 

-- 14、查询和“002”号的同学学习的课程完全相同的其他同学学号和姓名；

--  select count(1) from score where student_id=1;

-- select student_id,sname from score LEFT JOIN student on score.student_id=student.sid where student_id in (

--

--   select student_id from score where student_id !=1 GROUP BY student_id HAVING count(1)=(select count(1) from score where student_id=1)

--

-- )and course_id in(select course_id from score where student_id=1) GROUP BY student_id having count(1) =(select count(1) from score where student_id=1);

--

-- 

-- 15、删除学习“李平”老师课的SC表记录；

--  DELETE

-- FROM

--  	score

--  WHERE

-- 	course_id IN (

--  		SELECT

--  			cid

-- 		FROM

--  			course

-- 		LEFT JOIN teacher ON course.teacher_id = teacher.tid

--  		WHERE

--  			teacher.tname = "李平"

--  	)

--

--

-- 16、向SC表中插入一些记录，这些记录 要求符合以下条件：①没有上过编号“002”课程的同学学号；②插入“002”号课程的平均成绩； 

-- 插入表格 insert into tb(student_id,course_id,num) 映射里面加动态值 

--  select student_id from score where course_id!=2

-- SELECT

-- 	student_id,

-- 	2,

-- 	(

-- 		SELECT

-- 			AVG(num)

-- 		FROM

-- 			score

-- 		WHERE

-- 			course_id = 2

-- 	)

-- FROM

-- 	score

-- WHERE

-- 	course_id != 2 

-- 17、按平均成绩从低到高显示所有学生的“语文”、“数学”、“英语”三门的课程成绩，按如下形式显示： 学生ID,语文,数学,英语,有效课程数,有效平均分；

-- select student_id,

--    (select num from score as s2 where s2.student_id=s1.student_id and course_id=1)as 语文,

--    (select num from score as s2 where s2.student_id=s1.student_id and course_id=1)as 数学,

--    (select num from score as s2 where s2.student_id=s1.student_id and course_id=1)as 英语

-- from score as s1;

--

--

-- 18、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；

-- SELECT

-- 	course_id,

-- 	MAX(num),

-- 	min(num)

-- FROM

-- 	score

-- GROUP BY

-- 	course_id

-- 

-- 19、按各科平均成绩从低到高和及格率的百分数从高到低顺序；

-- 运用了 case when xx then xx else xx 判断语句

-- SELECT

-- 	course_id,

-- 	avg(num),

-- 	sum(

-- 		CASE

-- 		WHEN num < 60 THEN

-- 			0

-- 		ELSE

-- 			1

-- 		END

-- 	),

-- 	sum(1),

-- 	sum(

-- 		CASE

-- 		WHEN num < 60 THEN

-- 			0

-- 		ELSE

-- 			1

-- 		END

-- 	) / sum(1) AS jgl

-- FROM

-- 	score

-- GROUP BY

-- 	course_id

-- ORDER BY

-- 	avg(num) ASC,

-- 	jgl DESC;

-- 

--

-- 20、课程平均分从高到低显示（现实任课老师）；

-- 知识点 if(ISNULL(score.num),0,(score.num))   三元运算

-- select score.course_id,avg(num),teacher.tname

-- from score LEFT JOIN  course on score.course_id=course.cid

-- LEFT JOIN teacher on teacher.tid=course.teacher_id

-- GROUP BY score.course_id ORDER BY avg(num)desc;

--

--

-- 21、查询各科成绩前三名的记录:(不考虑成绩并列情况)

-- select num from score where course_id =1 ORDER BY num desc LIMIT 0,1;

-- select num from score where course_id =1 ORDER BY num desc LIMIT 1,1;

-- select num from score where course_id =1 ORDER BY num desc LIMIT 2,1;

-- select *from

-- (

-- select student_id,course_id,num,1,

-- (select num from score as s2 where s2.course_id=s1.course_id GROUP BY s2.num ORDER BY s2.num desc LIMIT 0,1),

-- (select num from score as s2 where s2.course_id=s1.course_id GROUP BY s2.num ORDER BY s2.num desc LIMIT 3,1)as cc

--

-- from score as s1

-- )as B

-- where B.num >B.cc

-- 

--

-- 22、查询每门课程被选修的学生数；

-- select course_id,count(1) from score GROUP BY score.course_id

-- 23、查询出只选修了一门课程的全部学生的学号和姓名；

-- SELECT

-- 	A.student_id,

-- 	student.sname

-- FROM

-- 	(

-- 		SELECT

-- 			student_id,

-- 			count(1)

-- 		FROM

-- 			score

-- 		GROUP BY

-- 			student_id

-- 		HAVING

-- 			count(1) = 1

-- 	) AS A

-- LEFT JOIN student ON A.student_id = student.sid

-- 

--

-- 24、查询男生、女生的人数；

-- SELECT

-- 	*

-- FROM

-- 	(

-- 		SELECT

-- 			count(gender) AS F

-- 		FROM

-- 			student

-- 		WHERE

-- 			gender = "男"

-- 	) AS A,

-- 	(

-- 		SELECT

-- 			count(gender) AS M

-- 		FROM

-- 			student

-- 		WHERE

-- 			gender = "女"

-- 	) AS B

--

--

-- 25、查询姓“张”的学生名单；

-- select *from student where sname like "张%"

--

-- 26、查询同名同姓学生名单，并统计同名人数；

--  SELECT

--  	sname,

--  	count(sname)

--  FROM

--  	student

-- GROUP BY

--  	sname 

-- 27、查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列；

-- select course_id,avg(if(isnull(num),0,num))as avg from score GROUP BY course_id ORDER BY avg asc,course_id desc;

	--

-- 28、查询平均成绩大于85的所有学生的学号、姓名和平均成绩；

-- SELECT

-- 	student_id,

-- 	sname,

-- 	avg(IF(isnull(num), 0, num))

-- FROM

-- 	score

-- LEFT JOIN student ON score.student_id = student.sid

-- GROUP BY

-- 	student_id

-- 

--

-- 29、查询课程名称为“生物”，且分数低于60的学生姓名和分数；

-- select *from score where num<60 and course_id=(select cid from course where cname="生物")

-- select student.sname,score.num from score LEFT JOIN course on score.course_id=course.cid

-- LEFT JOIN student on score.student_id=student.sid where score.num <60 and course.cname="生物"

--

-- 

-- 30、查询课程编号为003且课程成绩在80分以上的学生的学号和姓名；

-- SELECT

-- 	A.student_id,

-- 	student.sname

-- FROM

-- 	(

-- 		SELECT

-- 			*

-- 		FROM

-- 			score

-- 		WHERE

-- 			course_id = 3

-- 		AND score.num > 80

-- 	) AS A

-- LEFT JOIN student ON A.student_id = student.sid

--

	--

-- 31、求选了课程的学生人数

-- select student_id from score GROUP BY student_id

	--

-- 32、查询选修“张磊”老师所授课程的学生中，成绩最高的学生姓名及其成绩；

-- SELECT

-- 	sname,

-- 	num

-- FROM

-- 	score

-- LEFT JOIN student ON score.student_id = student.sid

-- WHERE

-- 	score.course_id IN (

-- 		SELECT

-- 			course.cid

-- 		FROM

-- 			course

-- 		LEFT JOIN teacher ON course.teacher_id = teacher.tid

-- 		WHERE

-- 			tname = '张磊老师'

-- 	)

-- ORDER BY

-- 	num DESC

-- LIMIT 1;

--

	--

-- 33、查询各个课程及相应的选修人数；

-- select course.cname,count(1)from score LEFT JOIN course on score.course_id=course.cid GROUP BY course_id;

-- 

	--

-- 34、查询不同课程但成绩相同的学生的学号、课程号、学生成绩；

-- SELECT DISTINCT

-- 	s1.course_id,

-- 	s2.course_id,

-- 	s1.num,

-- 	s2.num

-- FROM

-- 	score AS s1,

-- 	score AS s2

-- WHERE

-- 	s1.num = s2.num

-- AND s1.course_id != s2.course_id;

--

--

--

	--

-- 35、查询每门课程成绩最好的前两名；

-- select *from

-- (

-- select student_id,course_id,num,1,

-- (select num from score as s2 where s2.course_id=s1.course_id GROUP BY s2.num ORDER BY s2.num desc LIMIT 0,1),

-- (select num from score as s2 where s2.course_id=s1.course_id GROUP BY s2.num ORDER BY s2.num desc LIMIT 2,1)as cc

--

-- from score as s1

-- )as B

-- where B.num >B.cc

-- 

-- 36、检索至少选修两门课程的学生学号；

-- SELECT

-- 	student_id,

-- 	count(1)

-- FROM

-- 	score

-- GROUP BY

-- 	student_id

-- HAVING

-- 	count(1) > 1;

-- 37、查询全部学生都选修的课程的课程号和课程名；

-- SELECT

-- 	course_id,

-- 	count(1)

-- FROM

-- 	score

-- GROUP BY

-- 	course_id

-- HAVING

-- 	count(1) = (SELECT count(1) FROM student);

-- 38、查询没学过“李平”老师讲授的任一门课程的学生姓名；

-- SELECT

-- 	*

-- FROM

-- 	student

-- WHERE

-- 	sid NOT IN (

-- 		SELECT

-- 			student_id

-- 		FROM

-- 			score

-- 		WHERE

-- 			course_id IN (

-- 				SELECT

-- 					cid

-- 				FROM

-- 					course

-- 				LEFT JOIN teacher ON course.teacher_id = teacher.tid

-- 				WHERE

-- 					teacher.tname = "李平老师"

-- 			)

-- 	)

-- 39、查询两门以上不及格课程的同学的学号及其平均成绩；

-- 先对学生进行分组 对课程数进行统计，筛选分数小于60分

-- SELECT

-- 	student_id,

-- 	count(1)

-- FROM

-- 	score

-- WHERE

-- 	num < 60

-- GROUP BY

-- 	student_id

-- HAVING

-- 	count(1) > 2;

-- 40、检索“004”课程分数小于60，按分数降序排列的同学学号；

-- 运用了排序方法，order by xx desc方法排序

--

-- SELECT

-- 	student_id

-- FROM

-- 	score

-- WHERE

-- 	course_id = 4

-- AND num < 60

-- ORDER BY

-- 	num DESC

-- 

-- 41、删除“002”同学的“001”课程的成绩；

-- 运用了表格的删除功能 delete from score表格；

-- DELETE

-- FROM

-- 	score

-- WHERE

-- 	score.course_id = 1

-- AND score.student_id = 2

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