简单查询
创建students表
create table students(
id int not null unique primary key auto_increment,
name varchar(10) not null,
class varchar(10) not null,
grade int
)
添加数据
insert into students values(0,'老1','1班',80);
insert into students values(0,'老2','2班',90);
insert into students values(0,'老3','3班',86);
insert into students values(0,'老4','4班',70);
insert into students values(0,'老5','5班',89);
insert into students values(0,'老6','6班',93);
查询所有数据
select * from students
查询指定字段
select name,class from students
通过as给表起别名
select s.name,s.class from students as s
通过as给字段起别名
select name as 姓名,class as 班级 from students
条件查询
比较运算
select * from students where class='1班'
select * from students where grade>90
逻辑运算
and、or、not
select * from students where grade<90 and class!='1班'
模糊查询
like、%任意多个字符、_一个任意字符
select * from students where name like '老%'
select * from students where name like '%5'
范围查询
in表示非连续范围内、between...and...在一个连续范围内
select * from students where class in('1班','3班','4班')
select * from students where grade between 80 and 90
空判断
is null 与' '不同，' '为空字符串
select * from students where grade is null
select * from students where class=' '
insert into students(name,class,grade) values('老7','1班',null)
排序
order by 列1 asc|desc,列2 asc|desc
asc从小到大，升序，不写则为升序
desc从大到小，降序
select * from students order by grade
先按grade降序、再按id升序
select * from students order by grade desc,id
先按id升序、再按grade降序
select * from students order by id,grade desc
聚合函数
count
select count(*) from students
带字段的不统计为null的数据
select count(grade) from students
max、min、sum、avg
select max(grade) from students
select avg(grade) from students
分组
select sex,count(*) from students group by sex
select grade,count(*) from students group by grade
select class,grade,count(*) from students group by grade,class
分组后过滤
使用having，必须跟在group by之后
where是用于from后的数据筛选、having是对group by的结果进行筛选
获取部分行
索引0开始
select * from students order by grade limit 0,3
分页
每页显示m条数据，第n页的数据
select * from students limit (n-1)*m,m