数据库-and,or,in,distinct,like,@,is null,is not null,where,GROUP BY,聚合函数MAX,MIN,AVG

1.基本概念

SELECT  表上哪些列显示
*表示所有列 
//投影运算 指的是选择部分列数据
//友好列标题 SELECT Sno as 学号
//top关键字 SELECT Top 3 * from Student 只显示前3条数据
FROM 这个表来源
WHERE 这个表的情况，进行筛选

2.简单匹配查询

2.1 检索年龄为20岁的学生信息

select * from student where Sage=20

2.2 and

连接多个条件 
select * from student where ssex='女'and sage>21

2.3 or

表示达到一个条件即可
between and 在什么条件之间， not between and 取反条件
select sno,sname,ssex from student whrer sno between '2' and '4'

2.4 in

检索一系列的取值列表  
select * from teacher where tropt in（'教授','副教授'）

2.5 distinct

表示不检索出相同信息 
select distinct tropt from teacher

2.6 like

模糊检索 
select sname，sdept from student where sdept like'%学%'

2.7 @

一个字符串与一个下划线进行匹配 
select sname,sdept from student where emall like '__@%'

2.8 is null

字段为空
select *from student where low is null

2.9 is not null

字段不为空 
select *from student where high is not null

2.10 where

等于单个值 查询有某科目考试分数为48分的学生信息
select *from student   where sno=(select sno from sc where grade=48)

3.分组聚合查询

2.1 GROUP BY

进行分组 按照by之后的选择进行分组统计

2.2 实例

对course表，按照必修和选修进行分类，统计每种类别的课程数量
select xklb as 类别，count(cname) as 数量 from course group by xklb 
HAVING
ORDER BY ASC||DESC
ASC表示升序，DESC表示降序
select *from student order by sage desc
select *from student order by sage desc,sno asc在sage相同情况下按snow升序对比排列
COUNT 
count函数返回匹配行数
select count（*） from teacher where tropt=‘教授’

2.3 聚合函数MAX,MIN,AVG

显示教师的最大最小平均年龄
select max(tage),min(tage),avg(tage) from teacher 
指定条件求和 
select sum(credit) from course where xklb='必修' 对course表中的必修课的学分进行求和

2.4 混合应用

select smajor,ssex,count(sno) from student group by Smajor ,sex order by count(sno) desc
对student表，按照专业和性别进行分组，显示每个专业，每种性别的学生数量，按照学生数量的降序显示结果
select tropt,count(tropt) from teacher group by tropt having count(tropt)>=5
对teacher表，显示职称和对应的认识，要求只有统计人数大于等于5人才显示