• 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