【数据库】基础语法

select from where order by   > >= < <= != and or not
模糊匹配 like % _   
空值 null 
为空 is null 
在集合里 in (集合)   
存在 in 
排序 order by   asc  desc
取前n个 top(n) 
随机抽取 rand(), newid()
集函数  计数 count()  distinct -剔除重复  
求和 sum(数值列)  avg() max min  
分组 group by 列  
筛选 having

已知：
Student(Sno,Sname,Ssex,Sage,Sdept)
Course(Cno,Cname,Ccredit,Cpno)
SC(Sno,Cno,Grade)

• 查询挂科超过2门的同学的姓名

select Sname from Student where Sno in (
select Sno from SC where Grade <60 group by Sno having count(*) >=2) 

• 查询年龄最大的学生

select top 2 * from Student order by Sage

• 随机挑选10名学生 newid()

select top 10 * from Student order by newid()

• 挑选成绩排名前三的同学

select top 3 * from SC order by Grade desc

• 查询成绩全部优秀同学 (逆否命题：不存在成绩不优秀的选课，同时这个同学一定选课)

select * from Student where not Sno in(
select Sno from SC where Grade < 90) and Sno in(
select Sno from SC where Grade >=90) and not Sno in (
select Sno from SC where Grade is null)

• 模糊匹配 like %表示任意字符 _单个字符 escape 转义符

select * from Student where ssex ='女' and Sname like '刘%'

• IS或CS系的同学 in 集合

select * from Student where Sdept in('IS','CS')

• 统计每个系的女生人数并且查询女生数大于2的系名称

select count(*), Sdept from Student
where Ssex = '女' 
group by Sdept 
having count(*) >=2

• 年龄降序排列的前50%的同学

select top (select count(*) * 50/100  from Student)  * from Student 
order by Sage desc 

• 查询姓刘并且所有课程成绩均90分以上的女同学

select * from Student where  not Sno in(
select Sno from SC where Grade < 90) and Sno in(
select Sno from SC where Grade >=90) and not Sno in (
select Sno from SC where Grade is null) 
 and ssex ='女' and Sname like '刘%'

• 查询 保研的同学占比一半 英语成绩大于85，平均成绩前 15%同学 （4学分以上课程不挂科）

select * from Student where Sno in(
select top (select count(*) * 50/100  from Student)
  Sno from SC where Sno in( -- 英语85分以上 
   select Sno from SC where Grade >=85 and Cno in (select Cno from Course 
   where Cname = '英语')) and not Sno in (select Sno from SC where Grade <60 
   and Cno in (select Cno from Course where Ccredit >=4)) 
  group by Sno  order by avg(Grade) desc 
  )