create table student(  sid varchar(50),

                       sname varchar(50),

                       sage  varchar(50),

                       ssex  varchar(50)

                    );

insert into student( sid,sname,sage,ssex ) values('','zhaolei','1990-01-01','nan');

insert into student values('','qiandian','1990-12-21','nan');

insert into student values('','sunfeng','1990-05-20','nan');

insert into student values('','liyun','1990-08-06','nan');

insert into student values('','zhoumei','1991-12-01','nv'),

                          ('','wulan','1992-03-01','nv'),

                          ('','zhenzu','1989-07-01','nv'),

                          ('','wangju','1990-01-20','nv');

select * from student;

select sid,sname,sage,ssex from student;

select sid,sname,ssex from student;

=============================================================================================

1、修改表名

语法格式：alter table 旧表名 rename [to] 新表名;

注释：修改后example1表就不存在了，只存在名为user的新表，但是其内容是一致的，只是换了个名称.

alter table student rename to sww;

select * from student;

select * from sww;

alter table sww rename to student;

=====================================================================================================

2、修改字段名

语法格式：alter table 表名 change 旧属性名 新属性名 新数据类型;

注释：新数据类型指修改后的数据类型，如不需要修改，则将新数据类型设置成与原来一样

alter table student change sname sww varchar(50);

select * from student;

alter table student change sww sname varchar(50);

==============================================================================================

3、修改字段的数据类型

语法格式：alter table 表名 modify 属性名 数据类型;

注释：表名指所要修改数据类型的字段的表的名称；

      属性名指：所要修改数据类型字段的名称；

      数据类型指：修改后的新的数据类型

=========================================================================================================

4、修改字段的排列位置

语法格式：alter table 表名 modify 属性名1 数据类型 first|after 属性名2;

alter table  student modify sage varchar(50) first;

select * from student;

alter table  student modify sage varchar(50) after sname;

======================================================================================

5、增加字段

语法格式：alter table 表名 add 属性名1 数据类型 [完整性约束条件] [first | after 属性名2];

完整性约束条件：是可选参数，用来设置新增字段的完整性约束条件

first：是可选参数，其作用是将新增字段设置为表的第一个字的

after：是可选参数，其作用是将新增字段添加到“属性名2”所指的字段后

如果执行的SQL语句中没有“first”或者“after 属性名2”参数指定新增字段的位置，则新增字段默认为表的最后一个字段

alter table student add saddress varchar(50) after ssex;

select * from student;

==============================================================================================================

6、删除字段

删除字段是删除表中已经定义好的表中的某个字段,删除后其字段所属的数据都会被删除

语法格式：alter table 表名 drop 属性名;

alter table student drop saddress;

select * from student;

==========================================================================================================

select * from student;

select * from student where sid in('','','','');

select * from student where sid not in('','','','');

select * from student where  sid > 4;

select * from student where  sid != 4;

select * from student where sid between 4 and 6;

select * from student where sid not between 4 and 6;

select * from student where sname like '%n%';

select * from student where sname not like '%n%';

=====================================================================================================

select * from student;

select * from student order by sid desc;

select count(*) from student;

select sum(sid) from student;

select avg(sid) from student;

select max(sid) from student;

select min(sid) from student;

select count(*) from student group by ssex;

select sid as a, sname as b,sage as c, ssex as d from student;

select * from student limit 3;

select * from student limit 1,2;

====================================================================================

增加一个字段，插入数据，并更改数据

alter table student add saddress varchar(50) after ssex;

select * from student;

update student set saddress = 'jiangsu' where sid = '';

update student set saddress = 'jiangsu' where sid = '';

update student set saddress = 'shanghai' where sid = '';

update student set saddress = 'shanghai' where sid = '';

update student set saddress = 'shanghai' where sid = '';

update student set saddress = 'beijing' where sid = '';

update student set saddress = 'beijing' where sid = '';

update student set saddress = 'anhui' where sid = '';

insert into student ( sid,sname,sage,ssex ) values ('','shenweiwei','1989-10-18','nan');

update student set saddress = 'jiangsu' where sid = '';

一、将查询结果插入到表中

insert语句可以将一个表中查询到的数据插入到另外一个表中

语法格式：

insert into 表名1 (属性列表1) select 属性列表2 from 表名2 where 条件表达式;

表名1说明记录插入到哪个表中；

表名2表示记录是从哪个表中查询出来的；

属性列表1参数表示为哪些字段赋值；

属性列表2表示从表中查询出哪些字段的数据；

条件表达式参数设置了select语句的查询条件；

注意：使用这种方法时，必须保证属性列表1和属性列表2中的字段个数是一样的，而且每个对应字段的数据类型必须是一样的。

create table student2(  sid varchar(50),

                       sname varchar(50),

                       sage  varchar(50),

                       ssex  varchar(50),

                       saddress varchar(50)

                    );

select * from student2;

delete from student2;

insert into student2 ( sid,sname,sage,ssex,saddress ) values ('','fenglili2','1988-09-18','nv','jiangsu');

insert into student2 ( sid,sname,sage,ssex,saddress ) values ('','fenglili3','1988-08-18','nv','jiangsu');

insert into student2 ( sid,sname,sage,ssex,saddress ) values ('','fenglili4','1988-07-18','nv','shanghai');

insert into student ( sid,sname,sage,ssex,saddress ) values ('','fenglili2','1988-09-18','nv','jiangsu');

insert into student ( sid,sname,sage,ssex,saddress ) values ('','fenglili3','1988-08-18','nv','jiangsu');

insert into student ( sid,sname,sage,ssex,saddress ) values ('','fenglili4','1988-07-18','nv','shanghai');

delete from student where sid in('','','');

select * from student;

insert into student(sid,sname,sage,ssex) select sid,sname,sage,ssex from student2 where sid = '';

select * from student;

insert into student(sid,sname,sage,ssex,saddress) select sid,sname,sage,ssex,saddress from student2 where sid in ('','');

select * from student;

update student set saddress = 'jiangsu' where saddress is null;

select * from student;

================================================================================================================================

一、基本查询语句

select的基本语法格式如下：

select 属性列表 from 表名和视图列表

[ where 条件表达式1 ]

[ group by 属性名1 [ having 条件表达式2 ] ]

[ order by 属性名2 [ asc | desc ] ]

属性列表参数表示需要查询的字段名；

表名和视图列表参数表示从此处指定的表或者视图中查询数据，表和视图可以有多个；

条件表达式1参数指定查询条件；

属性名1参数指按照该字段的数据进行分组；

条件表达式2参数满足该表达式的数据才能输出；

属性名2参数指按照该字段中的数据进行排序；排序方式由asc和desc这两个参数指出；

asc参数表示升序，这是默认参数，desc表示降序；（升序表示从小到大）

对记录没有指定是asc或者desc，默认情况下是asc；

如果有where子句，就按照“条件表达式1”指定的条件进行查询；如果没有where子句，就查询所有记录；

如果有group by子句，就按照“属性名1”指定的字段进行分组，如果group by后面带having关键字，那么只有

满足“条件表达式2”中知道的条件才能输出。group by子句通常和count()、sum()等聚合函数一起使用；

如果有order by子句，就按照“属性名2”指定的字段进行排序，排序方式由asc和desc两个参数指出；默认情况下是asc；

查询结果不重复：distinct 关键字

select * from student;

select distinct ssex from student;

select distinct saddress from student;

分组查询：

select * from student group by ssex;

select * from student group by saddress;

group by 关键字与group_concat()函数一起使用，每个分组中指定字段值都显示出来：

select ssex,GROUP_CONCAT(ssex) from student group by ssex;

select ssex,GROUP_CONCAT(sname) from student group by ssex;

select saddress,GROUP_CONCAT(sname) from student group by saddress;

group by 关键字与集合函数一起使用：

select ssex,count(ssex) from student group by ssex;

select saddress,count(saddress) from student group by saddress;

group by 关键字与"having 条件表达式"一起使用，可以限制输出结果，只有满足条件表达式的结果才会显示：

select ssex,count(ssex) from student group by ssex;

select ssex,count(ssex) from student group by ssex having count(ssex) >6;

select saddress,count(saddress) from student group by saddress;

select saddress,count(saddress) from student group by saddress having count(saddress) >3;

注意：“having 表达式” 与 "where 表达式"都是用来限制显示的，但是两者起作用的地方不一样；

      "where 表达式"用于表或者视图，是表和视图的查询条件；

      “having 表达式”作用于分组后的记录，用于选择满足条件的分组。

group by 关键字与 with rollup 一起使用，会在所有记录的最后加上一条记录，这条记录是上面所有记录的总和。

select ssex,count(ssex) from student group by ssex;

select ssex,count(ssex) from student group by ssex with rollup;

select saddress,count(saddress) from student group by saddress;

select saddress,count(saddress) from student group by saddress with rollup;

select * from student;

select * from student limit 4;

select * from student limit 1,5;

=======================================================================================================

create table score ( xh int(50),

                     km varchar(50),

                     cj int(50)

                   );

select * from score;

insert into score values(1,'shuxue',80);

insert into score values(1,'yuwen',70);

insert into score values(1,'yingyu',40);

insert into score values(2,'shuxue',40);

insert into score values(2,'yuwen',60);

insert into score values(2,'yingyu',50);

insert into score values(3,'shuxue',60);

insert into score values(3,'yuwen',20);

insert into score values(3,'yingyu',90);

insert into score values(4,'shuxue',50);

insert into score values(4,'yuwen',60);

insert into score values(4,'yingyu',70);

select xh,sum(cj) from score where xh = 1;   查询此同学的总成绩；

select xh,sum(cj) from score where xh = 4;

select xh,sum(cj) from score group by xh;     查询每一个同学的各科总和成绩；

select km,max(cj) from score group by km;      查询各个科目的最高成绩；

select km,avg(cj) from score group by km;   查询每一科目的平均成绩；

select km,max(cj) from score group by km;    查询每一科目的最高成绩；

select km,min(cj) from score group by km;    查询每一科目的最低成绩；

=======================================================================================================================