mysql实战-常用查询
2017-01-14 16:47
- 创建数据库
Create DATABASE database-name
- 删除数据库
drop database dbname
- 创建新表
CREATE TABLE Persons
(
Id_P INT NOT NULL AUTO_INCREMENT,
LastName varchar(255),
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
- 删除表
drop table tabname
- 新增一列
Alter table tabname add column col type
- 插入数据
insert into table1(field1,field2) values(value1,value2)
- 删除数据
delete from table1 where id=1
- 更新
update table1 set field1=value1,field2=value2 where 范围
- 模糊查找
select * from table1 where field1 like '%value1%' or field2 like '%value2%'
下面说下多种查询
表结构如下:
- 学生资料表
- 学生的成绩表,sid就是学生资料表的外键
分数最高的学生信息,科目
方法一:左连接
select s.course as 科目,max(s.score1) as 最高分,c.name as 姓名 from score s left join student c on c.id = s.sid group by s.course;
方法二:子查询
select course as 科目,max(score1) as 最高分,(select name from student c where c.id=s.sid) as 姓名 from score s group by course;
科目评价为良好,性别为女的学生信息,科目
select s.score1,s.course,c.name,c.sex from score s join student c on s.sid=c.id where s.comment='良好' and c.sex='女';
最低分数性别为男和女的信息
select course,min(score1) as 最低分,c.sex,c.name from score s join student c on s.id=c.id and c.sex='男' or sex='女' group by course;
每科的前两名学生信息
select course,score1, c.name from score s1 left join student c on c.id=s1.sid where 2>(select count(*) from score s where s.course = s1.course and s.score1>s1.score1) order by s1.course,s1.score1
还有其他的两种方法可以取前两名学生数据:
方法二:select a.id,a.SName,a.ClsNo,a.Scorefrom Table1 a left join Table1 b on a.ClsNo=b.ClsNo and a.Score<b.Scoregroup by a.id,a.SName,a.ClsNo,a.Scorehaving count(b.id)<2order by a.ClsNo,a.Score desc
方法三:select *from Table1 awhere id in (select id from Table1 where ClsNo=a.ClsNo order by Score desc limit 2)order by a.ClsNo,a.Score desc
- 查询出2门及2门以上不及格者的平均成绩
select name,sum(score < 60) ,avg(score) from result group by name having sum(score<60) >=2;
distinct的用法
- 单列
select distinct name from A
- 多列
select distinct name, id from A
- COUNT结合
select count(distinct name) from A; --表中name去重后的数目, SQL Server支持,而Access不支持
count是不能统计多个字段的,若想使用,请使用嵌套查询,如下
select count(*) from (select distinct xing, name from B) AS M;
- distinct必须放在开头
select id, distinct name from A; --会提示错误,因为distinct必须放在开头