select 字段1，字段2 from 表1，表2 where 字段 group by 分组字段 having 字段 order by 字段 ASC|DESC limit m,n
一、查询所有
select * from student;
select name as 姓名 from student;
select name 姓名 from student;
二、条件查询
1.查询学生id等于1
selet * from student where student_id;
2.查询name字段为null
selet * from student where name is null;
3.查询name字段不为null
select * from student where name is not null;
4.查询name字段为空字符串
select * from student where name =‘‘;
5.查询name字段不为空字符串且不为null
select * from student where name !=‘‘;
6.查询city字段存在河南省和山东省的数据
select * from where city in(‘山东省‘,‘河南省‘);
7.查询age在25到30之间
select * from student where age between 25 and 30;
8.查询age不在20到30之间
select * from student where age not between 20 and 30;
9.查询第一个字是郭的数据
select * from student where name like ‘郭%‘;
10查询第一个字是郭的两个字得数据
select * from student where name like ‘郭_‘;
11查询存在郭的字得数据
select * from student where name like ‘%郭%‘;
三、分组查询
1.分组性别
select sex,count(*) from student group by sex;
2.分组性别前条件查询
select sex,count(*) from student where name !=‘李白‘ group by sex;
3.分组性别后条件查询
select sex,count(*) from student group by sex having count(*)>5;
4.查询后分组然后再条件查询
select sex,count(*) from student where name !=‘李白‘ group by sex having count(*)>5;
四、排序查询
1.降序排序
select * from student order by salary desc;
2.降序后再升序排序
select * from student order by salary desc,bonus asc;
五、聚合函数
count(*) sum(salary) avg(salary) min(salary) max(salary)
