原文https://www.cnblogs.com/wupeiqi/articles/5748496.html
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2、查询“生物”课程比“物理”课程成绩高的所有学生的学号；
思路：
获取所有有生物课程的人（学号，成绩） - 临时表
获取所有有物理课程的人（学号，成绩） - 临时表
根据【学号】连接两个临时表：
学号 物理成绩 生物成绩
然后再进行筛选
select
A.student_id,sw,ty
from
(
select
student_id,num
as
sw
from
score
left
join
course
on
score.course_id = course.cid
where
course.cname =
'生物'
)
as
A
left
join
(
select
student_id,num
as
ty
from
score
left
join
course
on
score.course_id = course.cid
where
course.cname =
'体育'
)
as
B
on
A.student_id = B.student_id
where
sw > if(
isnull
(ty),0,ty);
3、查询平均成绩大于60分的同学的学号和平均成绩；
思路：
根据学生分组，使用
avg
获取平均值，通过
having
对
avg
进行筛选
select
student_id,
avg
(num)
from
score
group
by
student_id
having
avg
(num) > 60
4、查询所有同学的学号、姓名、选课数、总成绩；
select
score.student_id,
sum
(score.num),
count
(score.student_id),student.sname
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
count
(tid)
from
teacher
where
tname
like
'李%'
select
count
(1)
from
(
select
tid
from
teacher
where
tname
like
'李%'
)
as
B
6、查询没学过“叶平”老师课的同学的学号、姓名；
思路：
先查到“李平老师”老师教的所有课ID
获取选过课的所有学生ID
学生表中筛选
select
*
from
student
where
sid
not
in
(
select
DISTINCT
student_id
from
score
where
score.course_id
in
(
select
cid
from
course
left
join
teacher
on
course.teacher_id = teacher.tid
where
tname =
'李平老师'
)
)
7、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名；
思路：
先查到既选择001又选择002课程的所有同学
根据学生进行分组，如果学生数量等于2表示，两门均已选择
select
student_id,sname
from
(
select
student_id,course_id
from
score
where
course_id = 1
or
course_id = 2)
as
B
left
join
student
on
B.student_id = student.sid
group
by
student_id
HAVING
count
(student_id) > 1
8、查询学过“叶平”老师所教的所有课的同学的学号、姓名；
同上，只不过将001和002变成
in
(叶平老师的所有课)
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；
同第1题
10、查询有课程成绩小于60分的同学的学号、姓名；
select
sid,sname
from
student
where
sid
in
(
select
distinct
student_id
from
score
where
num < 60
)
11、查询没有学全所有课的同学的学号、姓名；
思路：
在分数表中根据学生进行分组，获取每一个学生选课数量
如果数量 == 总课程数量，表示已经选择了所有课程
select
student_id,sname
from
score
left
join
student
on
score.student_id = student.sid
group
by
student_id
HAVING
count
(course_id) = (
select
count
(1)
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；
思路：
获取 001 同学选择的所有课程
获取课程在其中的所有人以及所有课程
根据学生筛选，获取所有学生信息
再与学生表连接，获取姓名
select
student_id,sname,
count
(course_id)
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”同学所有课的其他同学学号和姓名；
先找到和001的学过的所有人
然后个数 ＝ 001所有学科 ＝＝》 其他人可能选择的更多
select
student_id,sname,
count
(course_id)
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
having
count
(course_id) ＝ (
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；
个数相同
002学过的也学过
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
(course_id) = (
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
(course_id) = (
select
count
(1)
from
score
where
student_id = 1)
15、删除学习“叶平”老师课的score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.
name
=
'叶平'
)
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；
思路：
由于
insert
支持
inset
into
tb1(xx,xx)
select
x1,x2
from
tb2;
所有，获取所有没上过002课的所有人，获取002的平均成绩
insert
into
score(student_id, course_id, num)
select
sid,2,(
select
avg
(num)
from
score
where
course_id = 2)
from
student
where
sid
not
in
(
select
student_id
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
sc.student_id,
(
select
num
from
score
left
join
course
on
score.course_id = course.cid
where
course.cname =
"生物"
and
score.student_id=sc.student_id)
as
sy,
(
select
num
from
score
left
join
course
on
score.course_id = course.cid
where
course.cname =
"物理"
and
score.student_id=sc.student_id)
as
wl,
(
select
num
from
score
left
join
course
on
score.course_id = course.cid
where
course.cname =
"体育"
and
score.student_id=sc.student_id)
as
ty,
count
(sc.course_id),
avg
(sc.num)
from
score
as
sc
group
by
student_id
desc
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)
as
max_num,
min
(num)
as
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；
思路：
case
when
..
then
select
course_id,
avg
(num)
as
avgnum,
sum
(
case
when
score.num > 60
then
1
else
0
END
)/
count
(1)*100
as
percent
from
score
group
by
course_id
order
by
avgnum
asc
,percent
desc
;
20、课程平均分从高到低显示（现实任课老师）；
select
avg
(if(
isnull
(score.num),0,score.num)),teacher.tname
from
course
left
join
score
on
course.cid = score.course_id
left
join
teacher
on
course.teacher_id = teacher.tid
group
by
score.course_id
21、查询各科成绩前三名的记录:(不考虑成绩并列情况)
select
score.sid,score.course_id,score.num,T.first_num,T.second_num
from
score
left
join
(
select
sid,
(
select
num
from
score
as
s2
where
s2.course_id = s1.course_id
order
by
num
desc
limit 0,1)
as
first_num,
(
select
num
from
score
as
s2
where
s2.course_id = s1.course_id
order
by
num
desc
limit 3,1)
as
second_num
from
score
as
s1
)
as
T
on
score.sid =T.sid
where
score.num <= T.first_num
and
score.num >= T.second_num
22、查询每门课程被选修的学生数；
select
course_id,
count
(1)
from
score
group
by
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
student.sid, student.sname,
count
(1)
from
score
left
join
student
on
score.student_id = student.sid
group
by
course_id
having
count
(1) = 1
24、查询男生、女生的人数；
select
*
from
(
select
count
(1)
as
man
from
student
where
gender=
'男'
)
as
A ,
(
select
count
(1)
as
feman
from
student
where
gender=
'女'
)
as
B
25、查询姓“张”的学生名单；
select
sname
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
(1)
as
count
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“数学”，且分数低于60的学生姓名和分数；
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
*
from
score
where
score.student_id = 3
and
score.num > 80
31、求选了课程的学生人数
select
count
(
distinct
student_id)
from
score
select
count
(c)
from
(
select
count
(student_id)
as
c
from
score
group
by
student_id)
as
A
32、查询选修“杨艳”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
score.sid,score.course_id,score.num,T.first_num,T.second_num
from
score
left
join
(
select
sid,
(
select
num
from
score
as
s2
where
s2.course_id = s1.course_id
order
by
num
desc
limit 0,1)
as
first_num,
(
select
num
from
score
as
s2
where
s2.course_id = s1.course_id
order
by
num
desc
limit 1,1)
as
second_num
from
score
as
s1
)
as
T
on
score.sid =T.sid
where
score.num <= T.first_num
and
score.num >= T.second_num
36、检索至少选修两门课程的学生学号；
select
student_id
from
score
group
by
student_id
having
count
(student_id) > 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“叶平”老师讲授的任一门课程的学生姓名；
select
student_id,student.sname
from
score
left
join
student
on
score.student_id = student.sid
where
score.course_id
not
in
(
select
cid
from
course
left
join
teacher
on
course.teacher_id = teacher.tid
where
tname =
'张磊老师'
)
group
by
student_id
39、查询两门以上不及格课程的同学的学号及其平均成绩；
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；
select
student_id
from
score
where
num< 60
and
course_id = 4
order
by
num
desc
;
41、删除“002”同学的“001”课程的成绩；
delete
from
score
where
course_id = 1
and
student_id = 2
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