create table student( sid varchar(50),
sname varchar(50),
sage varchar(50),
ssex varchar(50)
);
insert into student( sid,sname,sage,ssex ) values('1','zhaolei','1990-01-01','nan');
insert into student values('2','qiandian','1990-12-21','nan');
insert into student values('3','sunfeng','1990-05-20','nan');
insert into student values('4','liyun','1990-08-06','nan');
insert into student values('5','zhoumei','1991-12-01','nv'),
('6','wulan','1992-03-01','nv'),
('7','zhenzu','1989-07-01','nv'),
('8','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('1','3','5','7');
select * from student where sid not in('1','3','5','7');
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 = '1';
update student set saddress = 'jiangsu' where sid = '2';
update student set saddress = 'shanghai' where sid = '3';
update student set saddress = 'shanghai' where sid = '4';
update student set saddress = 'shanghai' where sid = '5';
update student set saddress = 'beijing' where sid = '6';
update student set saddress = 'beijing' where sid = '7';
update student set saddress = 'anhui' where sid = '8';
insert into student ( sid,sname,sage,ssex ) values ('9','shenweiwei','1989-10-18','nan');
update student set saddress = 'jiangsu' where sid = '9';
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 ('11','fenglili2','1988-09-18','nv','jiangsu');
insert into student2 ( sid,sname,sage,ssex,saddress ) values ('12','fenglili3','1988-08-18','nv','jiangsu');
insert into student2 ( sid,sname,sage,ssex,saddress ) values ('13','fenglili4','1988-07-18','nv','shanghai');
insert into student ( sid,sname,sage,ssex,saddress ) values ('11','fenglili2','1988-09-18','nv','jiangsu');
insert into student ( sid,sname,sage,ssex,saddress ) values ('12','fenglili3','1988-08-18','nv','jiangsu');
insert into student ( sid,sname,sage,ssex,saddress ) values ('13','fenglili4','1988-07-18','nv','shanghai');
delete from student where sid in('11','12','13');
select * from student;
insert into student(sid,sname,sage,ssex) select sid,sname,sage,ssex from student2 where sid = '11';
select * from student;
insert into student(sid,sname,sage,ssex,saddress) select sid,sname,sage,ssex,saddress from student2 where sid in ('12','13');
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；
=======================================================================================================================
