建立表
請建立如下表，并建立相關限制。如圖1
操作表中的内容
1、自行建立測試資料；
2、查詢學生總人數；
3、查詢“生物”課程和“實體”課程成績都及格的學生id和姓名；
4、查詢每個年級的班級數，取出班級數最多的前三個年級；
5、查詢平均成績最高和最低的學生的id和姓名以及平均成績；
6、查詢每個年級的學生人數；
7、查詢每位學生的學号，姓名，選課數，平均成績；
8、查詢學生編号為“2”的學生的姓名、該學生成績最高的課程名、成績最低的課程名及分數；
9、查詢姓“李”的老師的個數和所帶班級數；
10、查詢班級數小于5的年級id和年級名；
11、查詢班級資訊，包括班級id、班級名稱、年級、年級級别(12為低年級，34為中年級，56為高年級)，示例結果如下；
12、查詢學過“張三”老師2門課以上的同學的學号、姓名；
13、查詢教授課程超過2門的老師的id和姓名；
14、查詢學過編号“1”課程和編号“2”課程的同學的學号、姓名；
15、查詢沒有帶過高年級的老師id和姓名；
16、查詢學過“張三”老師所教的所有課的同學的學号、姓名；
17、查詢帶過超過2個班級的老師的id和姓名；
18、查詢課程編号“2”的成績比課程編号“1”課程低的所有同學的學号、姓名；
19、查詢所帶班級數最多的老師id和姓名；
20、查詢有課程成績小于60分的同學的學号、姓名；
21、查詢沒有學全所有課的同學的學号、姓名；
22、查詢至少有一門課與學号為“1”的同學所學相同的同學的學号和姓名；
23、查詢至少學過學号為“1”同學所選課程中任意一門課的其他同學學号和姓名；
24、查詢和“2”号同學學習的課程完全相同的其他同學的學号和姓名；
25、删除學習“張三”老師課的score表記錄；
26、向score表中插入一些記錄，這些記錄要求符合以下條件：①沒有上過編号“2”課程的同學學号；②插入“2”号課程的平均成績；
27、按平均成績從低到高顯示所有學生的“國文”、“數學”、“英語”三門的課程成績，按如下形式顯示： 學生ID,國文,數學,英語,有效課程數,有效平均分；
28、查詢各科成績最高和最低的分：以如下形式顯示：課程ID，最高分，最低分；
29、按各科平均成績從低到高和及格率的百分數從高到低順序；
30、課程平均分從高到低顯示(現實任課老師)；
31、查詢各科成績前三名的記錄(不考慮成績并列情況)
32、查詢每門課程被選修的學生數；
33、查詢選修了2門以上課程的全部學生的學号和姓名；
34、查詢男生、女生的人數，按倒序排列；
35、查詢姓“張”的學生名單；
36、查詢同名同姓學生名單，并統計同名人數；
37、查詢每門課程的平均成績，結果按平均成績升序排列，平均成績相同時，按課程号降序排列；
38、查詢課程名稱為“數學”，且分數低于60的學生姓名和分數；
39、查詢課程連結描述編号為“3”且課程成績在80分以上的學生的學号和姓名；
40、求選修了課程的學生人數
41、查詢選修“王五”老師所授課程的學生中，成績最高和最低的學生姓名及其成績；
42、查詢各個課程及相應的選修人數；
43、查詢不同課程但成績相同的學生的學号、課程号、學生成績；
44、查詢每門課程成績最好的前兩名學生id和姓名；
45、檢索至少選修兩門課程的學生學号；
46、查詢沒有學生選修的課程的課程号和課程名；
47、查詢沒帶過任何班級的老師id和姓名；
48、查詢有兩門以上課程超過80分的學生id及其平均成績；
49、檢索“3”課程分數小于60，按分數降序排列的同學學号；
50、删除編号為“2”的同學的“1”課程的成績；
51、查詢同時選修了實體課和生物課的學生id和姓名；
GITHUB
可以從github直接下載下傳sql檔案
建立表
# 自行建立測試資料
-- create database school charset utf8;
use `school`;
# 建立班級表
DROP TABLE IF EXISTS `class`;
create table `class`(
`cid` int unsigned not null auto_increment ,
`caption` varchar(50) not null unique ,
`grade_id` int unsigned not null ,
PRIMARY KEY(`cid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC `class`;
# 建立學生表
DROP TABLE IF EXISTS `student`;
CREATE TABLE student(
`sid` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
`sname` VARCHAR(50) NOT NULL ,
`gender` ENUM('男','女') NOT NULL DEFAULT '男',
`class_id` INT UNSIGNED NOT NULL,
PRIMARY KEY (`sid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC `student`;
# 建立老師表
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher`(
tid INT UNSIGNED NOT NULL AUTO_INCREMENT ,
tname VARCHAR(50) NOT NULL ,
PRIMARY KEY (tid)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC `teacher`;
# 建立課程表
DROP TABLE IF EXISTS `course`;
CREATE TABLE `course`(
`cid` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
`cname` VARCHAR(50) NOT NULL ,
`teacher_id` INT UNSIGNED NOT NULL,
PRIMARY KEY (`cid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC course;
# 建立成績表
DROP TABLE IF EXISTS `score`;
CREATE TABLE `score`(
`sid` INT UNSIGNED NOT NULL AUTO_INCREMENT,
`student_id` INT UNSIGNED NOT NULL,
`course_id` INT UNSIGNED NOT NULL,
`score` INT UNSIGNED NOT NULL DEFAULT 0,
PRIMARY KEY (`sid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC `score`;
# 建立年紀表
DROP TABLE IF EXISTS `class_grade`;
CREATE TABLE `class_grade`(
`gid` INT UNSIGNED NOT NULL AUTO_INCREMENT,
`gname` VARCHAR(50) NOT NULL,
PRIMARY KEY (`gid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC `class_grade`;
# 班級班級任職表
DROP TABLE IF EXISTS `teach2cls`;
CREATE TABLE `teach2cls`(
`tcid` INT UNSIGNED NOT NULL AUTO_INCREMENT,
`tid` INT UNSIGNED NOT NULL,
`cid` INT UNSIGNED NOT NULL,
PRIMARY KEY (`tcid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
DESC `class_grade`;
-- 插入資料
-- 插入班級表
INSERT INTO `class`(`caption`, `grade_id`) VALUES
('一年一班', 1),
('二年一班', 2),
('三年二班', 3),
('一年二班', 1),
('二年二班', 2),
('三年一班', 3),
('四年一班', 4),
('五年一班', 5),
('六年一班', 6);
-- 插入學生表
INSERT INTO `student`(`sname`, `gender`, `class_id`) VALUES
('喬丹', '女', 1),
('艾費森','女',1),
('科比','男',2),
('亨利', '男', 2),
('alex', '女', 3),
('jenry', '女', 4),
('tom', '男', 3),
('mike', '男', 5),
('sam', '男', 8),
('den', '男', 9),
('張三', '男', 2),
('張五', '男', 4),
('jenry', '男', 1);
-- 插入老師表
INSERT INTO `teacher`(`tname`) VALUES
('張三'),
('李四'),
('王五'),
('賈六'),
('孫七');
-- 插入課程表
INSERT INTO `course`(`cname`, `teacher_id`) VALUES
('生物', 1),
('體育', 1),
('實體', 2),
('國文', 5),
('數學', 3),
('英語', 4);
-- 插入成績表
INSERT INTO `score`(`student_id`, `course_id`, `score`) VALUES
(1, 1, 60),
(1, 2, 59),
(2, 2, 99),
(2, 3, 80),
(2, 4, 50),
(3, 1, 100),
(3, 6, 89),
(4, 5, 40),
(6, 3, 70),
(10, 1, 80),
(7, 4, 20),
(11, 6, 71),
(11, 5, 100),
(13, 5, 80),
(1, 3, 70),
(2, 1, 90);
-- 插入年級表
INSERT INTO `class_grade`(`gname`) VALUES
('一年級'),
('二年級'),
('三年級'),
('四年級'),
('五年級'),
('六年級');
-- 插入班級任職表
INSERT INTO `teach2cls`(`tid`, `cid`) VALUES
(1, 1),
(1, 2),
(2, 1),
(3, 2),
(5, 1),
(4, 3),
(1, 3),
(2, 4),
(3, 6),
(5, 7),
(5, 8),
(4, 5);
50題
-- 1.查找學生總人數
SELECT COUNT(sid) as 'student_c' FROM student;
-- 2、查詢“生物”課程和“實體”課程成績都及格的學生id和姓名；
SELECT student.sid, student.sname, course.cname
FROM (student INNER JOIN score on student.sid = score.student_id)
INNER JOIN course on score.course_id = course.cid
WHERE cname = '生物' or cname = '實體' and score >= 60
ORDER BY sid;
-- 3、查詢每個年級的班級數，取出班級數最多的前三個年級；
SELECT class_grade.gname, COUNT(class.cid) AS 'class_c'
FROM class LEFT JOIN class_grade ON class.grade_id = class_grade.gid
GROUP BY class_grade.gid
ORDER BY class_c DESC
LIMIT 3;
-- 4、查詢平均成績最高和最低的學生的id和姓名以及平均成績；
SELECT student.sid, student.sname, AVG(score.score)
FROM score INNER JOIN student ON score.student_id = student.sid
GROUP BY score.student_id
HAVING AVG(score.score) IN (
(SELECT AVG(score.score) AS 'score_a'
FROM student INNER JOIN score ON student.sid = score.student_id
GROUP BY student.sid
ORDER BY score_a DESC
LIMIT 1),
(SELECT AVG(score.score) AS 'score_a'
FROM student INNER JOIN score ON student.sid = score.student_id
GROUP BY student.sid
ORDER BY score_a
LIMIT 1));
-- 5、查詢每個年級的學生人數；
SELECT class_grade.gname, COUNT(student.sid) AS 'student_c'
FROM (class RIGHT JOIN class_grade ON class.grade_id = class_grade.gid)
LEFT JOIN student ON class.cid = student.class_id
GROUP BY gname;
-- 6、查詢每位學生的學号，姓名，選課數，平均成績；
SELECT student.sid, sname, COUNT(course_id) AS 'course_c', IFNULL(AVG(score),0) AS 'score_a'
FROM student LEFT JOIN score ON student.sid = score.student_id
GROUP BY sname
ORDER BY sid;
-- 7、查詢學生編号為“2”的學生的姓名、該學生成績最高的課程名、成績最低的課程名及分數；
SELECT course.cname, score.score
FROM course INNER JOIN score ON course.cid = score.course_id
WHERE score IN (
(SELECT MAX(score)
FROM score
WHERE student_id = 2),
(SELECT MIN(score)
FROM score
WHERE student_id = 2));
-- 8、查詢姓“李”的老師的個數和所帶班級數；
SELECT COUNT(DISTINCT teacher.tname) AS 'teacher_c',COUNT(teach2cls.cid) AS 'class_c'
FROM teacher LEFT JOIN teach2cls ON teacher.tid = teach2cls.tid
WHERE teacher.tname LIKE '李%';
-- 9、查詢班級數小于5的年級id和年級名；
SELECT gid, class_grade.gname
FROM class RIGHT JOIN class_grade ON class.grade_id = class_grade.gid
GROUP BY class_grade.gname
HAVING COUNT(caption) < 5
ORDER BY gid;
-- 10、查詢班級資訊，包括班級id、班級名稱、年級、年級級别(12為低年級，34為中年級，56為高年級)，示例結果如下；
SELECT class.cid, class.caption, class_grade.gname,
CASE
WHEN class_grade.gid BETWEEN 1 AND 2 THEN '低年級'
WHEN class_grade.gid BETWEEN 3 AND 4 THEN '中年級'
WHEN class_grade.gid BETWEEN 5 AND 6 THEN '高年級'
END AS 'level'
FROM class INNER JOIN class_grade ON class.grade_id = class_grade.gid;
-- 11、查詢學過“張三”老師2門課以上的同學的學号、姓名；
SELECT sid, sname
FROM student
WHERE sid IN
(SELECT student_id
FROM score
WHERE course_id IN
(SELECT cid
FROM course
WHERE teacher_id IN
(SELECT tid
FROM teacher
WHERE tname = '張三'
)
)
GROUP BY student_id
HAVING COUNT(course_id) > 2);
-- 12、查詢教授課程超過2門的老師的id和姓名；
SELECT teacher.tid, teacher.tname
FROM teacher INNER JOIN course ON teacher.tid = course.teacher_id
GROUP BY teacher.tid
HAVING COUNT(course.cid) > 2;
-- 13、查詢學過編号“1”課程和編号“2”課程的同學的學号、姓名；
SELECT student.sid, student.sname
FROM student INNER JOIN score ON student.sid = score.student_id
WHERE score.course_id IN (1,2)
GROUP BY sname
HAVING COUNT(student.sid) = 2;
-- 14、查詢沒有帶過高年級的老師id和姓名；
SELECT tid, tname
FROM teacher
WHERE tid IN
(SELECT tid
FROM teach2cls
WHERE cid IN
(SELECT cid
FROM
(SELECT class.cid, class.caption, class_grade.gname,
CASE
WHEN class_grade.gid BETWEEN 1 AND 2 THEN '低年級'
WHEN class_grade.gid BETWEEN 3 AND 4 THEN '中年級'
WHEN class_grade.gid BETWEEN 5 AND 6 THEN '高年級'
END AS 'class_level'
FROM class INNER JOIN class_grade ON class.grade_id = class_grade.gid
) AS t1
WHERE t1.class_level != '高年級')
);
-- 15、查詢學過“張三”老師所教的所有課的同學的學号、姓名；
SELECT sid, sname
FROM student
WHERE sid IN
(SELECT student_id
FROM score
WHERE course_id IN
(SELECT cid
FROM teacher INNER JOIN course ON teacher.tid = course.teacher_id
WHERE teacher.tname = '張三')
);
-- 16、查詢帶過超過2個班級的老師的id和姓名；
SELECT teacher.tid, teacher.tname
FROM teacher INNER JOIN teach2cls ON teacher.tid = teach2cls.tid
GROUP BY teacher.tid
HAVING COUNT(cid) > 2;
-- 17、查詢課程編号“2”的成績比課程編号“1”課程低的所有同學的學号、姓名；
SELECT sid, sname
FROM student
WHERE sid IN
(SELECT t1.student_id
FROM
(SELECT student_id,score
FROM score
WHERE course_id = 2
) AS t1,
(SELECT student_id,score
FROM score
WHERE course_id = 1
) AS t2
WHERE
t1.score < t2.score);
-- 18、查詢所帶班級數最多的老師id和姓名；
SELECT tid, tname
FROM teacher
WHERE tid IN
(SELECT tid
FROM teach2cls
GROUP BY tid
HAVING COUNT(cid) =
(SELECT MAX(t1.count)
FROM
(SELECT tid, COUNT(cid) AS 'count'
FROM teach2cls
GROUP BY tid) AS t1));
-- 19、查詢有課程成績小于60分的同學的學号、姓名；
SELECT sid, sname
FROM student INNER JOIN score ON student.sid = score.student_id
WHERE score.score < 60;
-- 20、查詢沒有學全所有課的同學的學号、姓名；
SELECT student.sid, student.sname
FROM score INNER JOIN student ON score.student_id = student.sid
GROUP BY student_id
HAVING COUNT(course_id) >=
(SELECT COUNT(cid)
FROM course);
-- 21、查詢至少有一門課與學号為“1”的同學所學相同的同學的學号和姓名；
SELECT student.sid, student.sname
FROM score INNER JOIN student ON score.student_id = student.sid
WHERE course_id IN
(SELECT course_id
FROM score
WHERE score.student_id = 1)
GROUP BY student_id
-- 22、查詢至少學過學号為“1”同學所選課程中任意一門課的其他同學學号和姓名；
SELECT student.sid, student.sname
FROM score INNER JOIN student ON score.student_id = student.sid
WHERE course_id IN
(SELECT course_id
FROM score
WHERE score.student_id = 1)
GROUP BY student_id
HAVING student_id != 1; # 排除學号為1的同學
-- 23、查詢和“2”号同學學習的課程完全相同的其他同學的學号和姓名；
SELECT student.sid, student.sname
FROM score INNER JOIN student ON score.student_id = student.sid
WHERE student_id != 2
GROUP BY student_id
HAVING COUNT(course_id) =
(SELECT COUNT(course_id)
FROM score
WHERE student_id = 2)
-- 24、删除學習“張三”老師課的score表記錄；
DELETE
FROM score
WHERE course_id IN
(SELECT cid
FROM teacher INNER JOIN course ON teacher.tid = course.teacher_id
WHERE teacher.tname = '張三');
-- 25、向score表中插入一些記錄，這些記錄要求符合以下條件：①沒有上過編号“2”課程的同學學号；②插入“2”号課程的平均成績；
INSERT INTO score(student_id, course_id, score)
SELECT t1.sid, t1.cid, t2.avg_score
FROM
(SELECT student.sid, '2' AS cid
FROM student LEFT JOIN score ON student.sid = score.student_id
WHERE course_id != 2 OR ISNULL(course_id)) AS t1,
(SELECT IFNULL(AVG(score), 0) as 'avg_score'
FROM score
WHERE course_id = '2') AS t2
ORDER BY sid
-- 26、按平均成績從低到高顯示所有學生的“國文”、“數學”、“英語”三門的課程成績，按如下形式顯示： 學生ID,國文,數學,英語,有效課程數,有效平均分；
SELECT
sc.student_id,
(select score.score from score left join course on score.course_id = course.cid where course.cname = '國文' and score.student_id = sc.student_id) as yw,
(select score.score from score left join course on score.course_id = course.cid where course.cname = '數學' and score.student_id = sc.student_id) as sx,
(select score.score from score left join course on score.course_id = course.cid where course.cname = '英語' and score.student_id = sc.student_id) as yy,
COUNT(sc.course_id),
AVG(sc.score)
FROM score AS sc
GROUP BY sc.student_id
ORDER BY avg(sc.score) ASC;
-- 27、查詢各科成績最高和最低的分：以如下形式顯示：課程ID，最高分，最低分；
SELECT course.cid, MAX(score), MIN(score)
FROM score RIGHT JOIN course ON score.course_id = course.cid
GROUP BY course_id;
-- 28、按各科平均成績從低到高和及格率的百分數從高到低順序；
SELECT AVG(score) AS 'score_avg', CONCAT((CASE WHEN score.score > 60 THEN 1 ELSE 0 END) * 100, '%') AS 'percent'
FROM score
GROUP BY score.course_id
ORDER BY score_avg ASC, percent DESC;
-- 29、課程平均分從高到低顯示(現實任課老師)；
SELECT course.cname,teacher.tname, AVG(score.score) AS 'score_avg'
FROM (score RIGHT JOIN course ON score.course_id = course.cid)
INNER JOIN teacher ON course.teacher_id = teacher.tid
GROUP BY course_id
ORDER BY score_avg DESC;
-- 30、查詢各科成績前三名的記錄(不考慮成績并列情況)
SELECT course.cid, course.cname, s2.first_s, s2.second_s, s2.third_s
FROM
(SELECT course_id,
(SELECT score FROM score AS s2 WHERE s1.course_id = s2.course_id ORDER BY score DESC LIMIT 0,1) AS 'first_s',
(SELECT score FROM score AS s2 WHERE s1.course_id = s2.course_id ORDER BY score DESC LIMIT 1,1) AS 'second_s',
(SELECT score FROM score AS s2 WHERE s1.course_id = s2.course_id ORDER BY score DESC LIMIT 2,1) AS 'third_s'
FROM score AS s1
GROUP BY s1.course_id) AS s2 RIGHT JOIN course ON s2.course_id = course.cid;
-- 32、查詢每門課程被選修的學生數；
SELECT course.cid, course.cname, COUNT(score.student_id) AS 'student_c'
FROM score RIGHT JOIN course ON score.course_id = course.cid
GROUP BY course.cid;
-- 33、查詢選修了2門以上課程的全部學生的學号和姓名；
SELECT student.sid, student.sname
FROM score RIGHT JOIN student ON score.student_id = student.sid
GROUP BY student.sid
HAVING COUNT(score.course_id) > 2;
-- 34、查詢男生、女生的人數，按倒序排列；
SELECT gender ,COUNT(gender) AS 'gender_c'
FROM student
GROUP BY gender
ORDER BY gender_c DESC;
-- 35、查詢姓“張”的學生名單；
SELECT student.sid, student.sname, student.gender, class.caption
FROM student LEFT JOIN class ON student.class_id = class.cid
WHERE sname LIKE '張%';
-- 36、查詢同名同姓學生名單，并統計同名人數；
SELECT sname, count(sname) AS 'same_name_c'
FROM student
GROUP BY sname
HAVING COUNT(sname) > 1;
-- 37、查詢每門課程的平均成績，結果按平均成績升序排列，平均成績相同時，按課程号降序排列；
SELECT course.cid, course.cname, AVG(score) AS 'score_a'
FROM score RIGHT JOIN course ON score.course_id = course.cid
GROUP BY course_id
ORDER BY score_a, cid DESC;
-- 38、查詢課程名稱為“數學”，且分數低于60的學生姓名和分數；
SELECT student.sname, score.score
FROM (score RIGHT JOIN course ON score.course_id = course.cid)
INNER JOIN student ON score.student_id = student.sid
WHERE course.cname = '數學' AND score.score < 60;
-- 39、查詢課程編号為“3”且課程成績在80分以上的學生的學号和姓名；
SELECT student.sid, student.sname
FROM score INNER JOIN student ON score.student_id = student.sid
WHERE course_id = 3 AND score > 80;
-- 40、求選修了課程的學生人數
SELECT course.cid, course.cname, COUNT(student_id) AS 'student_c'
FROM score INNER JOIN course ON score.course_id = course.cid
GROUP BY course_id;
-- 41、查詢選修“王五”老師所授課程的學生中，成績最高和最低的學生姓名及其成績；
SELECT student.sname, t2.course_id, t2.score, t2.max_score, t2.min_score
FROM student
INNER JOIN
(SELECT score.student_id, score.course_id, score.score, t1.max_score, t1.min_score
FROM score,
(SELECT course_id, MAX(score) AS 'max_score' ,MIN(score) AS 'min_score'
FROM score
WHERE course_id IN
( SELECT cid
FROM course
INNER JOIN teacher ON course.teacher_id = teacher.tid
WHERE teacher.tname = '王五')
GROUP BY course_id) as t1
WHERE score.course_id = t1.course_id AND score.score IN (max_score, min_score)
) AS t2 ON student.sid = t2.student_id;
-- 42、查詢各個課程及相應的選修人數；
SELECT course.cid, course.cname, IFNULL(COUNT(score.student_id), 0) AS 'student_c'
FROM score RIGHT JOIN course ON score.course_id = course.cid
GROUP BY course.cid;
-- 43、查詢不同課時但成績相同的學生的學号、課程号、學生成績；
SELECT DISTINCT s1.student_id, s1.course_id, s1.score
FROM score as s1, score as s2
WHERE s1.student_id != s2.student_id AND s1.course_id != s2.course_id AND s1.score = s2.score;
-- 44、查詢每門課程成績最好的前兩名學生id和姓名；
SELECT student.sid, student.sname, t2.score,t2.course_id
FROM
(SELECT score.student_id, score.course_id, score.score
FROM
(select
s1.course_id,
(select s2.score from score as s2 where s1.course_id = s2.course_id order by s2.score desc limit 0,1) as first_score,
(select s3.score from score as s3 where s1.course_id = s3.course_id order by s3.score desc limit 1,1) as second_score
from
score as s1
GROUP BY s1.course_id) AS t1 INNER JOIN score ON t1.course_id = score.course_id
WHERE score.score IN (t1.first_score, t1.second_score)
) AS t2 INNER JOIN student ON student.sid = t2.student_id
ORDER BY t2.course_id , t2.score DESC;
-- 45、檢索至少選修兩門課程的學生學号；
SELECT student_id
FROM score
GROUP BY student_id
HAVING COUNT(course_id) >= 2;
-- 46、查詢沒有學生選修的課程的課程号和課程名；
SELECT course.cid, course.cname
FROM score RIGHT JOIN course ON score.course_id = course.cid
WHERE score.student_id IS NULL;
-- 47、查詢沒帶過任何班級的老師id和姓名；
SELECT teacher.tid, teacher.tname
FROM teach2cls RIGHT JOIN teacher ON teach2cls.tid = teacher.tid
WHERE teacher.tid IS NULL;
-- 48、查詢有兩門以上課程超過80分的學生id及其平均成績；
SELECT student_id, AVG(score)
FROM score
WHERE score.score > 80
GROUP BY student_id
HAVING COUNT(score.course_id) >= 2;
-- 49、檢索“3”課程分數小于60，按分數降序排列的同學學号；
SELECT student_id, score
FROM score
WHERE score.course_id = 3 AND score.score < 60
ORDER BY student_id DESC;
-- 50、删除編号為“2”的同學的“1”課程的成績；
DELETE FROM score
WHERE score.sid =
(SELECT t1.sid
FROM
(SELECT sid
FROM score AS s1
WHERE student_id = 2 AND course_id = 1) AS t1
);
-- 51、查詢同時選修了實體課和生物課的學生id和姓名；
SELECT sid, sname
FROM student
WHERE sid IN
(SELECT student_id
FROM score
WHERE course_id IN
(SELECT cid
FROM course
WHERE course.cname IN ('實體','生物'))
GROUP BY student_id
HAVING count(course_id) = 2
);