mysql——前面内容——前期整理笔记00

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;

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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;

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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);

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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

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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;

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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;

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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;

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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%';

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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;

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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;

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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;

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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；
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create table employee ( num int(50),
                        d_id int(50),
                        name varchar(50),
                        age int(50),
                        sex varchar(50),
                        homeadd varchar(50)
                       );

insert into employee values(1,1001,'zhangsan',26,'nan','beijing');
insert into employee values(2,1001,'lisi',24,'nv','hunan');
insert into employee values(3,1002,'wangwu',25,'nan','jiangsu');
insert into employee values(4,1004,'aric',15,'nan','yingguo');

select * from employee;

create table department ( d_id int(50),
                          d_name varchar(50),
                          functione varchar(50),
                          address varchar(50)
                        );

insert into department values(1001,'keyanbu','yanfachanpin','3lou5hao');
insert into department values(1002,'shengchanbu','shengchanchanp','5louyiceng');
insert into department values(1003,'xiaoshoubu','cehuaxiaoshou','1louxiaoshoudating');

select * from department;

select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id;

内连接查询：可以查询两个或者两个以上的表，当两个表中存在表示相同意义的字段时，可以通过该字段来连接这两个表；

             当该字段的值相等时，就查询出该记录。

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外连接查询：可以查询两个或者两个以上的表，外连接查询也需要通过指定字段来进行连。

            当该字段取值相等时，可以查询出该记。

            而且该字段取值不相等的记录也可以查询出来。

外连接包括：左连接、右连接

语法：

        select 属性列表 from 表名1 left | right join 表名2 on 表名1.属性名1 = 表名2.属性名2；

属性列表表示要查询的字段的名称，这些字段可以来自不同的表；

Left表示左连接查询；

rigth表示右连接查询；

on后面接的是连接的条件；

1、左连接查询

进行左连接查询时，可以查询出表名1所指的表中的所有记录。而表名2所指的表中，只能查询出匹配的记录

select * from employee;
select * from department;

select num,name,employee.d_id,age,sex,d_name,functione from employee left join department on employee.d_id = department.d_id;

2、右连接查询

进行右连接查询时，可以查询出表名2所指的表中的所有记录。而表名1所指的表中，只能查询出匹配的记录

select * from employee;
select * from department;

select num,name,employee.d_id,age,sex,d_name,functione from employee right join department on employee.d_id = department.d_id;

复合条件查询

在连接查询时，通过增加其他的限制条件，可以使查询结果更加准确

select * from employee;
select * from department;

select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id;

select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id and age > 24;

select num,name,employee.d_id,age,sex,d_name,functione from employee,department where employee.d_id = department.d_id order by age asc;

select 语句先按照内连接的方式从employee和department表中查询出数据。然后查询结果按照age字段从小到大的顺序进行排序。

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子查询

子查询是将一个查询语句嵌套在另外一个查询语句中，内层查询语句的查询结果，可以作为外来层查询语句提供查询条件。

1、带in关键词的子查询

select * from employee;
select * from department;

select * from employee where d_id in (select d_id from department );

select * from employee where d_id not in (select d_id from department );

2、带exists关键字的子查询

exists关键字表示存在，使用exists关键字时，内层查询语句不用返回查询的记录。而是返回一个真假值。

如果内层查询语句查询到满足条件的记录，就返回一个真值（true）；否则，返回一个假值（false）;

当返回值为true时，外层查询语句将进行查询；而返回false时，外层查询语句不进行查询或者查询不出任何记录。

select * from employee;
select * from department;

select * from employee where exists (select d_name from department where d_id = 1003);

select * from employee where exists (select d_name from department where d_id = 1004);

exists关键字可以与其他查询条件一起使用。条件表达式与exists关键字之间用and或者or来连接。

select * from employee where age > 24 and exists (select d_name from department where d_id = 1003);

select * from employee where age > 24 and exists (select d_name from department where d_id = 1004);

not exists与exists相反。

select * from employee where age > 24 and not exists (select d_name from department where d_id = 1003);

select * from employee where age > 24 and not exists (select d_name from department where d_id = 1004);

3、带比较运算符的子查询

create table schoarship ( levela int(50),
                          score int(50)
                         );

insert into schoarship(levela,score) values(1,90);
insert into schoarship values(2,80);
insert into schoarship values(3,70);

select * from schoarship;

create table computer_stu (  id int(50),
                             name varchar(50),
                             score int(50)
                           );

insert into computer_stu(id,name,score) values (1001,'lily',85);

insert into computer_stu(id,name,score) values (1002,'tom',91),
                                               (1003,'jim',87),
                                               (1004,'aric',77),
                                               (1005,'lucy',65),
                                               (1006,'andy',99),
                                               (1007,'ada',85),
                                               (1008,'jeck',70);

select * from computer_stu;

select id,name,score from computer_stu where score >= (select score from schoarship where levela = 1);

/* 查询获得一等奖学金的学生有哪些，第一个表为奖学金等级和最低分数*/

select d_id,d_name from department where d_id != (select d_id from employee where age = 24);

/*只有生产部和销售部没有年龄等于24岁的员工*/

4、带any关键字的子查询

any关键字表示满足其中任何一个条件。使用any关键字时，只要满足内查询语句返回的结果中的任何一个，就可以通过该条件来执行外层查询语句

select * from computer_stu where score >= any ( select score from schoarship );

/*结果显示，有7个人获得奖学金，只有id为1005的人没有，因为分数为65，不高于奖学金指定最低分数的任何一个*/

5、带all关键字的子查询

all关键字表示满足所有条件。使用all关键字时，只有满足内层查询语句返回的所有结果，才可以执行外层查询语句。

select * from computer_stu where score >= all ( select score from schoarship );

/*结果显示，只有两个人获得奖学金。因为这两个人的分数比所有奖学金要求的分数都高*/

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合并查询结果

合并查询结果 是将多个select语句的查询结果合并到一起

union关键字，数据库会将所有的查询结果合并到一起，然后除掉相同的记录；

union all关键字，只是简单的合并到一起

select d_id from department;

select d_id from employee;

select d_id from department union select d_id from employee;

select d_id from department union all select d_id from employee;

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为表和字段取别名

1、语法：表名 表的别名

select * from department d where d.d_id = 1001;

2、语法：属性名 [as] 别名

select d_id as department_id,d_name department_name from department;

数据库中，可以同时为表和字段取别名

select d_id as department_id,d_name department_name,d.functione,d.address from department d where d.d_id = 1001;

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