--单表查询
select * from student;
select Sno,Sname from Student;
--算术表达式
--查全体学生的姓名及其出生年份。
select Sno,Sname,Sage,2019-Sage from Student;
select year(now())
select Sno,Sname,Sage,year(now())-Sage from Student;
--字符串常量
select Sname,'年龄:',Sage from student;
--函数:字符串函数，日期函数，数学函数
select Sname,'年龄:',Sage ,sdept,LOWER(sdept) from student;
--列别名
select Sname,'年龄:' as 年龄,Sage from student;
select Sname as 姓名,Sage 年龄 from student;
--年龄+10
select Sname as 姓名,Sage+10 年龄 from student;
-- 刘晨 +10,other 不变
select Sname as 姓名,Sage+10 年龄 from student where sname='刘晨'
union
select Sname as 姓名,Sage 年龄 from student where sname<>'刘晨'
-- 》=19 成年，《19 未成年 ???
SELECT * from sc
select Sno from sc;
select all Sno from sc;
-- 消除取值重复的行
select DISTINCT sno from sc;
--查询满足条件的行:where
select * from student where sage =18
select * from student where sage >=19
select * from student where sage !=18
select * from student where sage <>18
--确定范围 [19,20]
select * from student where sage BETWEEN 19 and 20
--确定范围 [18,20)
select * from student where sage>=18 and sage<20
确定集合 谓词：IN <值表>, NOT IN <值表>
select * from student where sdept in ('CS','IS')
select * from student where sdept NOT in ('CS','IS')
字符匹配 谓词：[NOT] LIKE ‘<匹配串>’
匹配串为含通配符的字符串:% _
-- SDEPT 包含 S
SELECT * from student where sdept like '%S%'
SELECT * from student where sdept like 'S%'
SELECT * from student where sno like '%23'
SELECT * from student where sname like '张__'
SELECT * from student where sname like '张_'
涉及空值的查询
is null
查询没有先修课的课程
select * from course where cpno is null
select * from course where cpno is not null
多重条件查询
select * from student where ssex='男' and sage=20 and sdept='cs'
select * from student where ssex='男' and ssex='女'
select * from student where ssex='男' or sdept='ma'
select * from student where ssex='男' or sdept='ma' and sage between 18 and 19
21,25 23
21,23,25==>
21
23
25
select * from student where ssex='男' or (sdept='ma' and sage between 18 and 19)
select * from student where (ssex='男' or sdept='ma') and sage between 18 and 19
21,23,25==>
23
25
select * from student where ssex='男'
ORDER BY子句
select * from student order by sage desc
select * from student order by sage
select * from student order by sage asc
select * from student order by sage ,sno desc --年龄升序，学号降序
23,22,25,21
聚集函数
SELECT Count(*) from student
SELECT avg(Sage) from student
SELECT sum(Sage) from student
SELECT max(Sage) from student
SELECT min(Sage) from student
GROUP BY子句
select ssex 男女生,count(ssex) 人数 from student group by ssex
order by count(ssex) desc
select sage 年龄,count(sage) 人数 from student group by sage
order by 人数 desc
=======================================
男女生 人数
男 2
女 2
=======================================???
HAVING
--显示 各个年龄段 人数>=2 的统计结果
select sage 年龄,count(sage) 人数 from student group by sage
having count(sage)>=2