Mysql学习笔记

SQL structed query language

连接MYSQL服务器：mysql -uroot -proot

数据库的操作：创建，查看，修改，删除
*创建：
创建一个名称为mydb1的数据库。
create database mydb1;
创建一个使用utf-8字符集的mydb2数据库。
create database mydb2 character set utf8;
创建一个使用utf-8字符集，并带校对规则的mydb3数据库。
create database mydb2 character set utf8 collate utf8_general_ci;
*查看:
显示所有数据库
show databases;
显示创建数据库的语句信息
show create database mydb2;
*修改:
修改mydb1的字符集为gbk(不能修改数据库名)
alter database mydb1 character set utf8;
*删除:
删除数据库mydb2
drop database mydb1;

表的操作：创建，查看，修改，删除
*创建：
根据实体类Person创建表person
Person {
int id;
String name;
}
create table person(
id int,
name varchar(20)
);
mysql中的数据类型：
bit 1位　但可以指定位数，如：bit<3>
int 2字节 可以指定最大位数，如：int<4>　最大为4位的整数
float 2个字节　可以指定最大的位数和最大的小数位数，如：float<5,2> 最大为一个５位的数，小数位最多2位
double　4个字节　可以指定最大的位数和最大的小数位数，如：float<6,4> 最大为一个6位的数，小数位最多4位
char　必须指定字符数,如char(5) 为不可变字符　即使存储的内容为'ab',也是用5个字符的空间存储这个数据
varchar　必须指定字符数,如varchar(5) 为可变字符　如果存储的内容为'ab',占用2个字符的空间；如果为'abc',则占用3个字符的空间
text: 大文本(大字符串)
blob：二进制大数据　如图片，音频文件，视频文件
date: 日期　如：'1921-01-02'
datetime: 日期时间　如：'1921-01-02 12:23:43'
timeStamp: 时间戳，自动赋值为当前日期时间

创建一个员工表
create table employee(
id int,
name varchar(20),
sex bit,
birthday date,
entry_date date,
job varchar(20),
salary double,
resume text
);
*查看：
查看所有的表：
show tables;
查看指定表的创建语句
show create table employee;
显示指定表的结构：
desc employee;
*删除：
删除employee表
drop table employee;
*修改表：
增加一个字段：alter table worker add column height double;
修改一个字段：alter table worker modify column height float;
删除一个字段：alter table worker drop column height;
更改表名：rename table employee to worker;
修改表的字符集:alter table worker character set gbk;

表数据的CRUD
*C(create增加数据) Insert语句
新建Employee表并表中添加一些记录
create table employee(
id int,
name varchar(20),
sex bit,
birthday date,
salary double,
entry_date date,
resume text
);
insert into employee(name,id,sex,birthday,salary,entry_date,resume) values('张三',1,1,'1983-09-21',15000,'2012-06-24','一个大牛');
insert into employee(id,name,sex,birthday,salary,entry_date,resume) values(2,'李四',1,'1984-09-21',10000,'2012-07-24','一个中牛');
insert into employee(id,name,sex,birthday,salary,entry_date,resume) values(3,'王五',0,'1985-09-21',7000,'2012-08-24','一个小牛');

*U(update更新数据) Update语句
将所有员工薪水都增加500元。
update employee set salary=salary+500;
将王五的员工薪水修改为10000元，resume改为也是一个中牛
update employee set salary=10000,resume='也是一个中牛' where name='王五';
*D(drop删除数据) Delete语句
删除表中姓名为王五的记录。
delete from employee where name='王五';
删除表中所有记录。
delete from employee; --可以有条件，但删除所有记录差了一点
使用truncate删除表中记录。
truncate employee;--无条件 效率高

*R(Retrieve查找数据) Select语句
准备环境：
create table student(
id int,
name varchar(20),
chinese int,
english int,
math int
);

insert into student(id,name,chinese,english,math) values(1,'何东',80,85,90);
insert into student(id,name,chinese,english,math) values(2,'权筝',90,95,95);
insert into student(id,name,chinese,english,math) values(3,'何南',80,96,96);
insert into student(id,name,chinese,english,math) values(4,'叶坦',81,97,85);
insert into student(id,name,chinese,english,math) values(5,'何西',85,84,90);
insert into student(id,name,chinese,english,math) values(6,'丁香',92,85,87);
insert into student(id,name,chinese,english,math) values(7,'何北',75,81,80);
insert into student(id,name,chinese,english,math) values(8,'唐娇',77,80,79);
insert into student(id,name,chinese,english,math) values(9,'任知了',95,85,85);
insert into student(id,name,chinese,english,math) values(10,'王越',94,85,84);

查询表中所有学生的信息。
select * from student;
查询表中所有学生的姓名和对应的英语成绩。
select name,english from student;
过滤表中重复数据。
select english from student;
select DISTINCT english from student;
select DISTINCT english,name from student;

select english+chinese+math from student;
select english+chinese+math as 总分 from student;
select name,english+chinese+math as 总分 from student;

在所有学生英语分数上加10分特长分。
select name,english+10 from student;
统计每个学生的总分。
select english+chinese+math from student;
使用别名表示学生分数
select name,english+chinese+math as 总分 from student;
select name,english+chinese+math 总分 from student;

查询姓名为何东的学生成绩
select * from student where name='何东';
查询英语成绩大于90分的同学
select * from student where english>90;
查询总分大于250分的所有同学
select * from student where english+chinese+math>250;

查询英语分数在 85－95之间的同学。
select * from student where english>=85 and english<=95;
select * from student where english between 85 and 95;
查询数学分数为84,90,91的同学。
select * from student where math=84 or math=90 or math=91;
select * from student where math in(84,90,91);
查询所有姓何的学生成绩。
select * from student where name like '何%';
查询数学分>85，语文分>90的同学。
select * from student where math>85 and chinese>90;

对数学成绩排序后输出。
select * from student order by math;
对总分排序后输出，然后再按从高到低的顺序输出
select * from student order by math+chinese+english desc;
对姓何的学生成绩排序输出
select * from student where name like '何%' order by math+chinese+english desc;
select name, math+chinese+english from student where name like '何%' order by math+chinese+english desc;

统计一个班级共有多少学生？
select count(*) from student;
统计数学成绩大于90的学生有多少个？
select count(*) from student where math>90;
统计总分大于250的人数有多少？
select count(*) from student where math+chinese+english>250;

统计一个班级数学总成绩？
select sum(math) from student;
统计一个班级语文、英语、数学各科的总成绩
select sum(math), sum(chinese), sum(english) from student;
统计一个班级语文、英语、数学的成绩总和
select sum(math+chinese+english)from student;
select sum(math)+sum(chinese)+sum(english) from student;

求一个班级数学平均分？
select avg(math) from student;
求一个班级总分平均分
select avg(math+chinese+english)from student;
select avg(math)+avg(chinese)+avg(english) from student;

求班级最高分和最低分
select max(math+chinese+english),min(math+chinese+english) from student;

查出各个班的总分，最高分
准备环境
给表添加一个字段：alter table student add column class_id int;
更新表：
update student set class_id=1 where id<=5;
update student set class_id=2 where id>5;
select sum(math+chinese+english),max(math+chinese+english) from student group by class_id;

查询出班级总分大于1300分的班级ID
select class_id from student group by class_id having sum(math+chinese+english)>1300;

表的约束　　
*定义主键约束　primary key:不允许为空，不允许重复

*定义主键自动增长　auto_increment

*定义唯一约束　unique

*定义非空约束　not null

*定义外键约束　constraint ordersid_FK foreign key(ordersid) references orders(id)

*删除主键：alter table tablename drop primary key ;

数据的备份与恢复：
*准备数据：
create database mydb1;
use mydb1;
create table user(
id int,
name varchar(20)
);
insert into user(id,name) values(1,'Tom');
select * from user;
*备份：
mysqldump -uroot -p mydb1>d: est.sql;--备份数据库里的数据

*恢复；
mysql -uroot -p mydb1<d: est.sql
source d: est.sql(在mysql客户端上执行)

多表查询与子查询：
准备环境：
部门表
CREATE TABLE DEPARTMENT(
department_id int primary key auto_increment,
name varchar
);
职位表
CREATE TABLE LOCATION (
location_id int PRIMARY KEY auto_increment,
name varchar
);
员工表
CREATE TABLE EMPLOYEES(
employee_id int PRIMARY KEY auto_increment,
department_id int,
location_id int,
first_name varchar,
last_name varchar,
salary double,
hire_date date
);

向department表中添加数据
insert into department(name) values ('人事');
insert into department(name) values ('财务');
insert into department(name) values ('后勤');
insert into department(name) values ('公关');
insert into department(name) values ('研发');

向location表中添加数据
insert into location(name) values ('总经理');
insert into location(name) values ('经理');
insert into location(name) values ('主管');
insert into location(name) values ('组长');
insert into location(name) values ('职员');
向employees表中添加数据
insert into employees(department_id,location_id , first_name, last_name, salary,hire_date)
values (1,1, 'A', 'z', 50000, '2005-02-21');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (1,2, 'B', 'x', 20000, '2009-03-21');
insert into employees(department_id,location_id , first_name, last_name, salary,hire_date)
values (2,3, 'C', 'v', 10000, '2009-08-23');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (4,2, 'D', 'n', 30000, '2004-09-28');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (3,5, 'E', 'm', 3000, '2009-04-11');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,5, 'F', 'l', 5000, '2008-03-11');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,3, 'G', 'p', 20000, '2005-05-09');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,4, 'H', 'o', 8000, '2006-07-21');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,5, 'I', 'u', 6000, '2006-09-21');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,5, 'J', 'y', 5500, '2007-08-21');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,5, 'K', 't', 6500, '2006-12-21');
insert into employees(department_id,location_id, first_name, last_name, salary,hire_date)
values (5,1, 'L', 'r', 100000, '2001-05-21');
练习：
--多表查询：查出姓z的员工的所有信息
select * from employees e, department d, location l
where e.[department_id]=d.[department_id] and e.[location_id]=l.[location_id] and e.[last_name]='z';
--单行子查询: 查出所有工资大于平均工资的员工信息
select * from employees where salary >(select avg(salary) from employees)
--多行子查询: 查出所有所有姓z的部门名称
select name from department where department_id in (select department_id from employees where last_name='z');