1:创建表,修改表,添加列
/*创建基本表Student*/
create table Student(
Sno char(5) not null unique, /*定义的学号不能为空,并且是唯一的值。*/
Sname char(20),
Ssex char(2),
Sage int,
Sdept Char(15)
);
/*修改基本表Student*/
/*添加新列Scome*/
alter table Student Add Scome DATE;
/*修改数据类型*/
alter table Student MODIFY Sage SMALLINT;
/*删除学号的唯一标识*/
alter table Student DROP UNIQUE(Son);
/*删除表*/
drop table Student;
/*创建索引*/
CREATE CLUSTER INDEX Stusname ON Student(Sname);
/*删除索引*/
drop index Stusname;
2:基本的查询
/*查询*/
select Sname,Sage from Student;
select Sname,Sage,Sdept from Student;
select * from Student;
select Sname,1996-Sage from Student;
select Sname,'Year of Birth:',1996-Sage,LOWER(Sdept) from Student;
select Sname NAME,'Year of Birth:'BIRTH,1996-Sage BIRTHDAY,LOWER(Sdept) DEPARTMENT FROM Student; /*为列创建别名*/
3:复杂的查询
/*消除重复行*/
select DISTINCT Sno from Student;
/*查询满足条件的元组*/
select Sname from Student where Sdept="CD";
select Sname,Sage from Student where Sage>=20;
alter table Student add Grade int;
select DISTINCT Sno from Student where Grade<60;
/*确定范围-----------between and */
select Sname,Sage,Sdept from Student where Sage between 20 and 23;
/*确定集合-----------IN('','','')*/
select Sname,Ssex from Student where Sdept IN('IS','MA','CS');
/*字符匹配-----------LIKE+++( %:代表任意长度)( _ :代表单个字符)*/
select Sname,Sage from Student where Sname LIKE 'A%';
SELECT Sname ,Sage from Student where Sname LIKE 'A_';
/*字符匹配-------里面的转义字符-----ESCAPE'\':表示\为转义字符*/
/*查询条件是“A_”此时这里的_不是代表一个字符,只是单纯的表示下划线而已。因为语句前面有转义字符。*/
select Sname from Student where Sage LIKE 'A\__' ESCAPE '\';
/*涉及空值的查询*/
select Sname,Sage from Student where Grade IS NULL; /*查询成绩为空的学生*/
4:更复杂查询
/*多重条件查询*/
select Sname from Student where Sdept='cd' and Sage>20;
select Sname from Student where Sdept='cd' or Sage>20;
/*对查询结果进行排序*/
select Sname Grade from Student where Sage>20 order by Sage DESC;
select * from Student order by Sage DESC;
/*使用集函数*/
select count(*) from Student; /*求总个数*/
select count(distinct Sno) from Student;
select avg(Sage) from Student where Sname='ahui'; /*avg:求平均值*/
select Sname,count(Sage) from Student group by Sname;
select MAX(Sage) from Student where Sno='1'; /*最大值*/
5:连接查询
/*连接查询*/
/*等值的查询*/
select Student.*,SC.* from Student,SC where Student.Sno=SC.Sno;
/*自然连接两个表*/
select Student.Sno,Sname,Ssex,Sage,Sdept,Cno,SC.Grade from Student,SC where Student.Sno=SC.Sno;
/*将表Student进行了重新命名,为两个名字,从而进行对自己的查询*/
select FIRS.Sage,SECO.Sno from Student FIRS,Student SECO where FIRS.Sno=SECO.Sage;
/*外连接*/
select Student.Sno,Sname,Sage,Sdept,Ssex,Cno,SC.Grade from Student,SC where Student.Sno=SC.Sno(*);
/*复合条件连接--------就是利用and来进行操作*/
select Student.Sno,Sname from Student,SC where Student.Sno=SC.Sno and SC.Cno='2' and Student.Sage=2;
6: 嵌套查询
这个查询是将最里面的查询结果当作外面查询的条件来进行查询的,执行顺序是从里到外执行的。注意:最里面的查询(子查询的select语句中不能使用ORDER BY子句,ORDER BY子句只能对最终的查询结果进行排序)
/*嵌套查询---就是把一个查询的结果当做另一个查询的条件来进行查询*/
select Sname from Student where Sno
IN(
select Sno from SC where Cno='2'
);
/*--01:带有IN的子查询*/
select Sno,Sname,Sdept from Student where Sdept IN( select Sdept from Student where Sname='ahui');
/*--02:带有比较运算符的子查询*/
select Sno,Sname,Sdept from Student where Sdept=( select Sdept from Student where Sname='ahui');
7:集合查询
每一个select语句都能获得一个或一组元组,若要把多个select语句的结果合并为一个结果,可用集合操作来完成。集合操作主要包括并操作(UNION)交操作(INTERSECT),差操作(MINUS).
使用UNION将多个查询结果合并起来,形成一个完整的查询结果时,系统会自动去掉重复的元组;
注意的是参加UNION操作的各数据项数目必须相同,对应项的数据类型也要必须相同。
/*集合查询*/
/*查询计算机系的学生及年龄不大于19学生----是个并集*/
select * from Student where Sdept='CS' UNION select * from Student where Sage<=19;
select Sno from SC where Cno='1' UNION select Sno from SC where Cno='2';
8:数据更新
/*插入数据*/
insert into Student values('05','aaa','男','25','3','2015-11-26','3');
insert into SC(Sno,Cno) values('06','3');
insert into Deptage(Sdept,Avgage) select Sdept,AVG(Sage) from Student GROUP BY Sdept;
/*修改数据*/
update Student set Sage=233 where Sno='01';
update Student Set Sage=Sage+1;
update SC set Grade=1 where '1'=( select Sdept from Student where Student.Sno=SC.Sno);
/*删除数据*/
delete from Student where Sno='01';