例题：
先来讲讲not in 和not exists的区别，再开始今天的例题（和in，exists相反）
not in内外表做笛卡尔积，然后按照条件查询，没有用到索引
not exists是对外表进行循环，每次循环再对内表进行查询，先查的外表的内容，不是子查询的内容，并且依然用到了表上的索引
两者最大的区别是：not in只能返回一个字段
如：
select week6s.SNO 一门课程没选的学生学号
from week6s
where week6s.SNO not in(
select week6sc.SNO
from week6sc
group by week6sc.SNO)select week6s.SNO 一门课程都没选修的学生学号
from week6s
where not exists(
select *
from week6sc
where week6s.SNO=week6sc.SNO
)not in能表示的not exists也能表示（其实所有带in，比较运算符，any，all的子查询都能用exists表示），并且注意观察，两者引用的方式也不同，返回的结果是：
再着重讲一下两者的引用过程
not in相当于笛卡尔积的运算过程，找到符合not in前的一个字段的条件，不能是name，id not in
exists子查询的返回结果只返回真值或假值，所以给出列明没有实际的意义
对于exists，内层循环结果非空，外层where返回真值
对于not exists，内层循环结果非空，外层where返回假值
select *
from week6sc
where week6s.SNO=week6sc.SNO内层循环返回结果非空，所以not exists 为false，这里的记录在exists会被记录下来
补充：‘=’和‘in’
当确切知道内层循环返回单值时，可用比较运算符（‘>’'<''=''!='）
select sno,sname,sdept
from student
where(select sdept from student where sname='刘')=sdept在sql server中这条语句不会报错，但是sql标准要求子查询一定要跟在比较符之后，所以这条语句是错的。
1）检索至少选修一门课程的学生学号
select week6s.SNO 至少选修一门课程的学生学号
from week6s,week6sc
group by week6s.SNO,week6sc.SNO
having week6s.SNO=week6sc.SNO
2）检索一门课程都没选修的学生学号
select week6s.SNO 一门课程没选的学生学号
from week6s
where week6s.SNO not in(
select week6sc.SNO
from week6sc
group by week6sc.SNO)
3）检索至少选修两门课程的学生学号
select week6sc.SNO 至少选修两门课程的学生学号
from week6sc
group by week6sc.SNO
having count(week6sc.SNO)>1
这里也可以检索所有选了课的学生-选了小于两门课的学生，思路相同只是写起来比较繁琐
4）检索恰好选修一门课程的学生学号
select week6sc.SNO 选修了一门课程的学生学号
from week6s,week6sc
group by week6s.SNO,week6sc.SNO
having week6s.SNO=week6sc.SNO
and week6sc.SNO not in(
select week6sc.SNO
from week6sc
group by week6sc.SNO
having count(week6sc.SNO)>1
)
5）检索至少选修三门课程的学生学号
select week6sc.SNO 至少选修三门课程的学生学号
from week6sc
group by week6sc.SNO
having count(week6sc.SNO)>2
6）检索全部课程都选的学生学号（不存在一门课，成绩表中找不到他这门课的成绩记录）
select week6s.SNO 选修了全部课程的学生学号
from week6s
where not exists(
select * from week6c
where not exists(
select * from week6sc
where week6c.CNO=week6sc.CNO
and week6sc.SNO=week6s.SNO
)
)
7）检索选了S2所选全部课程的学生学号（同理，不存在一门课S2选了，而学生x没选）
select distinct sc1.SNO 选了S2所选的全部课程的学生学号
from week6sc sc1
where not exists(
select *
from week6sc sc2
where sc2.SNO='S2'
and not exists(
select *
from week6sc sc3
where sc3.SNO=sc1.SNO
and sc3.CNO=sc2.CNO
)
)
法2：
select distinct SNO
from week6sc
where CNO>=all(
select CNO
from week6sc
where SNO='s2'
)
