MySQL的基本操作

    
修改一个数据库的字符集    
alter database 数据库名 character set 字符集名;

修改一个数据库的校对规则
alter database 数据库名 collate 校对规则名;

删除一个数据库
drop database 数据库名;

练习:
1.将数据库mydb2的字符集改成utf8
2.将数据库mydb3的校对规则改成utf8_bin
3.删除mydb1数据库

创建数据表
create table 数据表名(
    列名 列类型,
    列名 列类型,
    列名 列类型
);

MySQL中的数据类型:
MySQL中的数据类型和Java中的数据类型并不是完全一一对应的关系.

数值类型
tinyint
smallint
MEDIUMINT
int                (最常用)
bigint



时间和时间戳的区别:
date:只包含日期,如果传入数据的时候,没有指定值,将是null
timestamp:包含日和时间,如果传入数据时,没有指定值,将是当前的系统时间

字符串类型:
char:(character)定长,例如:指定长度为100,但是实际传入的值只有20,剩余的字符用空格补全.
varchar:(variable character):可变长度,例如:指定长度为100,但是实际传入的值只有20,真实存储的就是20个字符

创建一个没有约束的员工表:

create table employee(
    id int,
    name varchar(20),
    gender varchar(10),
    birthday date,
    entry_date date,
    job varchar(50),
    salary double,
    resume text
);

查看表信息:
show create table employee;//显示建表信息
desc employee;//格式化显示

单表约束:
非空约束:not null
    name varchar(20) not null

唯一约束:unique
    id int unique
    在MySQL中,唯一列,允许有null值,null并不等于null
    
主键约束:primary key
    相当于非空约束 + 唯一约束
    uid int primary key auto_increment 
    auto_increment //表示此列的值如果不传入的话,系统自动赋值,在前一条记录基础上加1(通常用在int型主键上)

练习:
创建表employee2,加上各种约束

create table employee2(
    id int primary key auto_increment,
    name varchar(10) not null,
    gender varchar(10) not null,
    birthday date not null,
    entry_date date not null,
    job varchar(50) not null,
    salary double not null,
    resume text
);


删除表:
drop table tbl_name;


修改表:
添加列:
alter table employee2
add email varchar(20) not null;

删除列:
alter table employee2
drop email;

修改列:
alter table employee2
modify job varchar(40);

alter table employee2
change salary money float not null;

修改表名:
alter table employee2
rename to emp;

rename table emp to employee2;

alter table employee2 drop money;

alter table employee2 drop resume;


插入数据到数据表
insert into employee2 (id,name,gender,birthday,entry_date) values(1,'tom','male','2010-10-10','2018-3-4');

把一个表中的所有列都显示出来
select * from employee2;

insert into employee2 (id,name,gender,birthday,entry_date,job) values(2,'toms','male','2000-10-10','2018-2-5','boss');

当插入的列是一个表的所有列的时候,此时有简化写法:
insert into employee2 values(3,'toms','male','2005-10-10','2018-2-10','clear');

insert into employee2 values(4,'toms','male','2005-10-10','2018-2-10');//列数量不匹配

insert into employee2 values(null,'toms','male','2005-10-10','2018-2-10',null);//主键列可以传null,系统自动维护

//删除数据(记录)//删除表:drop 删除列:drop
delete from 表名 [where id = 2];

    
    delete from employee2 where id = 2;
    delete from employee2 ;

更新数据:update
update 表名 set 列名 = 值 [where记录过滤条件];

    update employee2 set gender = 'female';
    update employee2 set gender = 'male' where id = 1;

查询:select
select 列名1,列名2... from 表名;

    select * from emp;
    select id,name from emp;

中文乱码
insert into employee2 values(null,'张飞','男','2010-10-10','2018-2-10',null);

第一种解决方法:
临时:
mysql --default-character-set=gbk -uroot -proot;

修改配置文件
my.ini中修改
default-character-set=gbk
重启服务生效


更新练习:
练习:
1.将tom的id改成50
    update employee2 set id = 50 where name = 'tom';
    
2.将tyson的id改成60,job改成HR
    update employee2 set id = 60,job = 'HR' where name = 'tyson'; 
    
3.将toms的salary在原有基础上增加1000
    update employee2 set salary = salary + 1000 where name = 'toms';
    
4.把所有人的salary增加500
    update employee2 set salary = salary + 500;

truncate和delete的区别
truncate相当于delete不加where控制条件(清空表)
truncate的操作是:先把表删除(drop table ...),然后重新创建一个一样的表.
delete是一条一条数据的删除

练习:
1.删除表中id=2的记录
    delete from employee2 where id = 2;
    
2.删除表中所有记录
    delete from employee2 ;
    
3.使用truncate删除所有记录
    truncate employee2;

查询操作
select * from employee;
select name , id from employee;

select name ,id from stu where id > 3;//把id>3的记录的name和id列的值显示
select distinct name from stu;

create table stu(
    id int primary key auto_increment,
    name varchar(10),
    ch int not null,
    math int not null,
    team varchar(10)
);

insert into stu values(null,'tom',70,70,'first');
insert into stu values(null,'tom',80,80,'first');
insert into stu values(null,'toms',90,90,'first');

insert into stu values(null,'tyson',70,70,'second');
insert into stu values(null,'tyson',80,80,'second');
insert into stu values(null,'toy',90,90,'second');

insert into stu values(null,'tyson',70,70,'thrid');
insert into stu values(null,'tyson',80,80,'thrid');
insert into stu values(null,'toy',90,90,'thrid');

//查询出的列可以进行运算
select math + 10 from stu;

//别名
select math + ch as sum from stu;
select math + ch sum from stu;//as 可以省略


select * from stu where id > 3;
select * from stu where id <= 5;
select * from stu where id != 5;
select * from stu where id <> 5;


//选择一组和二组的记录
select * from stu where team in('first','second');
select * from stu where team = 'first' or team = 'second';

//区间
select * from stu where ch >= 70 and ch <= 85;
select * from stu where ch between 70 and 85;


//模糊查询
select * from stu where name like 'to%';
%占位符,表示任意多个字符
select * from stu where name like 'to_';
_占位符,表示一个任意字符

select * from stu where name like '%m%';

练习:
1.查询成绩在80-90之间的记录:between and
    select * from stu where ch between 80 and 90;
    
2.查询70,90分的记录
    select * from stu where ch in(70,90);
    
3.查询所有姓张的同学的记录
    insert into stu values(null,'张三',80,90,'first',null);
    insert into stu values(null,'张三丰',80,90,'first',null);
    insert into stu values(null,'张飞',80,90,'first',null);
    select * from stu where name like '张%';
    
4.查询所有名字为两个字的记录
    select * from stu where name like '__';

//排序order by
select name,ch from stu order by ch asc;//按照ch列升序排序,asc可以省略
select name,ch from stu order by ch;

select name,ch from stu order by ch desc;
    
练习:(使用测试数据的表stu)
1.查询各个学生总成绩,并按总分从高到底排序
    select ch + math from stu order by ch + math desc;
    select ch + math sum from stu order by sum desc;
    
2.查询学生成绩,先按ch升序,ch相同,按math降序排序
    select * from stu order by ch asc, math desc;
    
3.将所有姓张的同学的ch成绩降序排列    
    select ch from stu where name like '张%' order by ch desc;
    
聚合函数
count
查询某列有多少行?
select count(name) from stu;
select count(*) from stu;    //查询表中的记录条数,不论某列是否有null值

查询stu中math成绩>80的记录条数
select count(*) from stu where math > 80;
    
查询stu中总成绩>150的记录条数
select * from stu where (ch + math) > 150;
select count(*) from stu where (ch + math) > 150;
    

sum
查询stu中math的总成绩
select sum(math) from stu;    
    
查询各科总成绩
select sum(ch),sum(math) from stu;    
    
查询整个班级总成绩
两种实现方式:
    1.先求出个人的总成绩,再加起来
        select sum(ch + math) from stu;
        select sum(ifnull(ch,0) + ifnull(math,0)) from stu;
        
    2.先求出整个班级单科总成绩,在加起来
        select sum(ch) + sum(math) from stu;
    
当记录中某列有null值,进行算术运算,结果都为null.    
如果使用聚集函数,就会把null值当0
解决办法:使用ifnull函数
    
查询math的平均分
    select sum(math) / count(*) from stu;
    
查询math的平均分
    select avg(math) from stu;
    
查询整个班级的平均分
    select avg(ifnull(math,0) + ifnull(ch,0)) from stu;
    //select avg(sum(ch) + sum(math)) from stu; 出错!!!

求math最高分
    select max(math) from stu;

求ch最低分
    select min(ch) from stu;//排除null值
    
    
create table orders(
    id int,
    product varchar(20),
    price double
);

insert into orders values(1,'洗衣机',900);
insert into orders values(2,'洗衣机',900);
insert into orders values(3,'电视',700);
insert into orders values(4,'电视',900);
insert into orders values(5,'吹风机',20);
insert into orders values(6,'游戏机',9);
    
//求每种产品的销售总额
select product , sum(price) from orders group by product;    
select product , sum(price) as sum from orders group by product order by sum desc;
    
//对分组后的数据再次进行过滤 (where)    
select product , sum(price) as sum from orders group by product where sum > 1000;//不能用where进行过滤
select product , sum(price) as sum from orders group by product having sum > 1000;
对group by分组之后的结果再次进行过滤,使用关键字having    
    
    
统计单价在100以上的产品销售总额大于1000的,降序排序
select product,sum(price) sum from orders where price > 100 group by product having sum > 1000 order by sum desc;    
s f w g h o    

笛卡尔积
表A                    表B
1 aa1                1  bb1
2 aa2                2  bb2
                    3  bb3

select * from a,b;
1 aa1 1  bb1
1 aa1 2  bb2
1 aa1 3  bb3
2 aa2 1  bb1
2 aa2 2  bb2
2 aa2 3  bb3

create table dept(
    did int primary key auto_increment,
    dname varchar(20)
);

create table emp(
    eid int primary key auto_increment,
    ename varchar(20),
    dno int
);

insert into dept values(null,'财务部');
insert into dept values(null,'研发部');
insert into dept values(null,'后勤部');

insert into emp values(null,'toms',1);
insert into emp values(null,'tyson',1);
insert into emp values(null,'tom',2);
insert into emp values(null,'lucy',3);
insert into emp values(null,'lily',4);

//内连接
select * from emp inner join dept on emp.dno = dept.did;
select * from emp,dept where emp.dno = dept.did;  (常用)