数据库系统

SQL语言简单查询

带分号 字符串用单引号 相等判断 =

创建数据库

//定义表ddl create table Student(S# char(8) not null,Sname char(10), Ssex char(2),Sage integer,D# char(2),Sclass char(6)); //添加元组，行信息dml insert into Student(S#,Sname,Ssex,Sage,D#,Sclass) values('23123','张思','女',20,'03','4545');

简单查询

select * from Student; select S#,Sname,Ssex,Sage,D# from Student; select * from Student where Sage<20; select distinct S# from SC where score>80; select distinct S# from SC where score>80 order by S# desc; select Sname from Student where Sname not like '%(1,多个字符) _(单个字符,2个表示一个汉字)\%';

多表查询：多表做笛卡尔积后where

select Sname from Student,SC where Student.S#=SC.S# and SC.C#='001' order by score desc; //使用别名来与自己做笛卡尔积 select t1.Tname as Teacher1,t2.Tname as Teacher2 from Teacher t1,Teacher t2 where t1.salary>t2.salary; //单表既学过001又学过002课,别名连接

子查询：利用select更新

//select结果当做多条value insert into st(s#,sname) select s#,sname from student where sname like '%伟'; //删除所有元组 delete from student; delete from student where Sage<20; update student set sage =sage+10 where sage<20;

数据库定义修正和撤销

alter table Student add Saddr char[40],PID char[18]; alter table Student modify Saddr char[50]; alter table Student drop unique(S#); drop table Student; use databasename; close databasename;