Basic SQL sentence

create database ST;
use st;
create table Student(
Sno varchar(10) primary key ,
Sname varchar(15) ,
Ssex varchar(2) ,
Sage int ,
Sdept varchar(10)
);
create table Course(
Cno varchar(10) primary key ,
Cname varchar(15) not null,
Cpo varchar(10) ,
Ccredit int 
foreign key (Cpo) references Course(Cno),
);
--if  you want to study Cno,you must study Cpo first
--so Cpo must come from Cno ,it`s a foreign  key
create table SC(
Sno varchar(10) ,
Cno varchar(10),
Grade int,
foreign key (Sno) references Student(Sno),
foreign key (Cno) references Course (Cno),
);
insert into Student values(‘200215121‘,‘李勇‘,‘男‘,20,‘CS‘);
insert into Student values(‘200215122‘,‘刘晨‘,‘女‘,19,‘CS‘);
insert into Student values(‘200215123‘,‘王敏‘,‘女‘,18,‘MA‘);
insert into Student values(‘200215125‘,‘张立‘,‘男‘,19,‘IS‘);


---now can`t insert cpo so cpo is 5n167n6
insert into Course(Cno,Cname,Ccredit) values(‘1‘,‘数据库‘,4);
insert into Course(Cno,Cname,Ccredit) values(‘2‘,‘数学‘,2);
insert into Course(Cno,Cname,Ccredit) values(‘3‘,‘信息系统‘,4);
insert into Course(Cno,Cname,Ccredit) values(‘4‘,‘操作系统‘,3);
insert into Course(Cno,Cname,Ccredit) values(‘5‘,‘数据结构‘,4);
insert into Course(Cno,Cname,Ccredit) values(‘6‘,‘数据处理‘,2);
insert into Course(Cno,Cname,Ccredit) values(‘7‘,‘PASCSL语言‘,4);



insert into SC values(‘200215121‘,‘1‘,92);
insert into SC values(‘200215121‘,‘2‘,85);
insert into SC values(‘200215121‘,‘3‘,88);
insert into SC values(‘200215122‘,‘2‘,90);
insert into SC values(‘200215122‘,‘3‘,80);

--edit table 
alter table student add Sgrade int;
alter table student drop column Sgrade; 
--edit the data type of column
alter table student alter column Sname varchar(15);
--alter varchar(15) to varchar(20)
alter table course alter column Cname varchar(20);
alter table student alter column Sname varchar(20);
--insert 
insert into course(Cpo)values(‘1‘);

select Sno,Sname
from Student
where(Ssex=‘男‘);

update course set Cpo =‘5‘ where Cno=‘1‘;

select *
from Course

update Course set Cpo=‘‘ where Cno=‘2‘
update Course set Cpo=‘1‘ where Cno=‘3‘
update Course set Cpo = ‘6‘ where  Cno=‘4‘
update Course set Cpo =‘7‘ where Cno=‘5‘
--can`t updata null
update Course set Cpo=‘‘ where Cno=‘6‘
update Course set Cpo=‘6‘ where Cno=‘7‘


--lower function 
select Sname,LOWER(Sdept)
from Student;

select Sname,Sdept
from Student

select Sno 
from SC;
--the distinct key can cancel the repeated result,distinct coresponding ALL ,and the default is all
select DIStinct Sno
from SC

select Sname,Sage
from Student
where Sage>=20;


select Sname,Sno
from Student
where Sno=(select distinct Sno
from SC
where Grade<85);

select Grade 
from SC
--between and is a open section it include 19 and 20 
select Sname,Sdept,Sage
from Student
where Sage BEtween 19 and 20;

select Sname,Ssex,Sdept
from Student
where Sdept IN(‘CS‘,‘MA‘);


select *
from Student;

select *
from Student
Where Sage not between 19 and 20 ;


--LIKE 
--[not] like ‘<String>‘ string  include ? / *
--if  String  did`nt include any wildcard ,it can be instead of by ‘=‘
use st;
go
select *
from Student
where Sno like‘200215121‘;
--one chinese word instead of two string position
select Sname
from Student
--where Sname like ‘李_‘
where Sname like ‘李%‘;

select Sname
from Student
where Sname not like ‘李%‘;

--transforlate _ & to common symbol use escape
-- this sentence instruct that the char follow ‘\‘ is just a common symbol
select Cno
from Course
where Cname like ‘DB\_Design‘escape ‘\‘ ;

update SC  set Grade=NULL where Cno=‘1‘;
-- null query 
select Sno,Cno
from SC
where Grade is null;

select *
from SC;


select *
from Student
where Ssex=‘女‘ and Sage <21 ;
-- in just equals many ‘or‘

--order by desc ->>down asc ->>up

select *
from Student
--order by Sage DESC ;
order by Sage ASC ;
 
 
 alter table SC  add Constraint chk_Grade check(Grade>0 and Grade <100);
 alter table SC drop constraint chk_Grade ;
 alter table Student add loc int ;
 alter table Student alter  column loc varchar(20) ; 
 alter table Student drop column loc ;
 
 --distinct cancel repeated values 
 
 /*count ->>count the num of row 
count( column) ->> count the num of values in column 
 sum(column ) ->> count 
 */
 select COUNT(*)
 from Student