MySQL

基本操作

关于数据库

//创建数据库
create database h_test;        
//查看数据库
show databases;  
//查看数据库信息    
show create database h_test;
//修改数据库的编码,可使用上一条语句查看是否修改成功
alter database h_test default character set gbk collate gbk_bin;      
//删除数据库
drop database h_test;
//综上,可以直接创建数据库且设置编码方式
CREATE DATABASE h_test DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
//首先选定操作的数据库
use h_test;
//创建表student
create table student(
  id  int(11),
  name  varchar(20),
  age int(11)
);
//查看数据表
show tables;
//查看数据表信息,后面加上参数/G可使结果更加美观
show create table student;
//查看表的的字段信息
desc student;
//修改表名
alter table student rename [to] h_student;
//修改字段名
alter table h_student change name stu_name varchar(20);
//修改字段的数据类型
alter table h_student modify id int(20);
//添加字段
alter table h_student add grade float;
//删除字段
alter table h_student drop grade;
//修改字段的位置
alter table h_student modify stu_name varchar(20) first;
alter table h_student modify id int(11) after age;
//删除数据表
drop table h_student;

表的约束

索引

//创建索引
//一.创建表的时候创建索引
create table 表名(
        字段名 数据类型[完整性约束条件],
        ...
        字段名 数据类型,
        [UNIQUE|FULLTEXT|SPATIAL] INDEX|KEY
  );
//1-1.创建普通索引
create table test1(
  id  INT,
  name VARCHAR(20),
  age INT,
  INDEX (id)
);
//可以插入一条数据,查看索引是否被使用
explain select * from test1 where id=1 \G;
//1-2.创建唯一性索引
create table test2(
  id  INT,
  name VARCHAR(20),
  age INT,
  UNIQUE INDEX unique_id(id asc)
);
//1-3.创建全文索引
create table test3(
  id  INT,
  name VARCHAR(20),
  age INT,
  FULLTEXT INDEX fulltext_name(name)
)ENGINE=MyISAM;
//1-4.创建单列索引
create table test4(
  id  INT,
  name VARCHAR(20),
  age INT,
  INDEX single_name(name(20))
);
//1-5.创建多列索引
create table test5(
  id  INT,
  name VARCHAR(20),
  age INT,
  INDEX multi(id,name(20))
);
//1-6.创建空间索引
create table test6(
  id  INT,
  space GEOMETRY NOT NULL,
  SPATIAL INDEX sp(space)
)ENGINE=MyISAM;
---------------------------------------------------
//二.使用create index语句在已经存在的表上创建索引
//首先新建一个表,这个表没有索引
create table student(
  id int,
  age int,
  name varchar(20),
  intro varchar(40),
  g GEOMETRY NOT NULL
)ENGINE=MyISAM;
//2-1.创建普通索引
create index index_id on student(id);
//2-2.创建唯一性索引
create unique index uniqueidx on student(id);
//2-3.创建单列索引
create index singleidx on student(age);
//2-4.创建多列索引
create index mulitidx on student(name(20),intro(40));
//2-5.创建全文索引
create fulltext index fulltextidx on student(name);
//2-6.创建空间索引
create spatial index spatidx on student(g); 
//三.使用alter table语句在已经存在的表上创建索引
//删除student表,重新创建
drop table student;
create table student(
  id int,
  age int,
  name varchar(20),
  intro varchar(40),
  space GEOMETRY NOT NULL
)ENGINE=MyISAM;
//3-1.创建普通索引
alter table student add index index_id(id);
//3-2.创建唯一性索引
alter table student add unique uniqueidx(id);
//3-3.创建单列索引
alter table student add index singleidx (age);
//3-4.创建多列索引
alter table student add index multidx(name(20),intro(40));
//3-5.创建全文索引
alter table student add fulltext index fulltextidx(name);
//3-6.创建空间索引
alter table student add spatial index spatidx(space);

//删除索引,有下面两种方式
//1.使用alter table删除索引fulltextidx
alter table student drop index fulltextidx;
//2.使用drop index删除索引spatidx
drop index spatidx on student;

添加数据

//重新建立表student
drop table student;
create table student(
  id int,
  name varchar(20) not null,
  grade float
);
//插入一条数据,也可以少某个字段的同时也少对应的数据
insert into student(id,name,grade) values(1,'howie',70);
//也可以不指定字段名,但要注意顺序
insert into student values(2,'howie',80);
//也可以这样添加数据
insert into student set id=3,name="howie",grade=90;
//同时添加多条数据
insert into student values
(4,'howie',80),
(5,'howie',80),
(6,'howie',80);

更新数据

//更新id=1的数据
update student set name="howie1",grade=60 where id=1;
//批量更新,如果没有where子句,会更新表中所有对应数据
update student set grade=100 where id<4;

删除数据

//删除id=6的数据
delete from student where id=6;
//批量删除数据
delete from student where id>3;
//删除所有数据,DDL(数据定义语言)语句 truncate table student也可以删除表内所有数据
delete from student;

单表查询和多表操作

单表查询

//建立表student
create table student(
  id int not null auto_increment,
  name varchar(20) not null,
  grade float,
  primary key(id)
);
//插入数据
insert into student (name,grade) values
("howie1",40),
("howie1",50),
("howie2",50),
("howie3",60),
("howie4",70),
("howie5",80),
("howie6",null);
//查询全部
select * from student;
//查询某个字段
select name from student;
//条件查询,查询id=2学生的信息
select * from student where id=2;
//in关键字查询,也可以使用not in
select * from student where id IN(1,2,3);
//between and关键字查询
select * from student where id between 2 and 5;
//空值(NULL)查询,使用IS NULL来判断
select * from student where grade is null;
//distinct关键字查询
select distinct name from student;
//like关键字查询,查询以h开头,e结尾的数据
select * from student where name like "h%e";
//and关键字多条件查询,or关键字的使用也是类似
select * from student where id>5 and grade>60;

高级查询

//聚合函数
//count()函数,sum()函数,avg()函数,max()函数,min()函数
select count(*) from student;
select sum(grade) from student;
select avg(grade) from student;
select max(grade) from student;
select min(grade) from student;
//对查询结果进行排序
select * from student order by grade;
//分组查询
//1.单独使用group by分组
select * from student group by grade;
//2.和聚合函数一起使用
select count(*),grade from student group by grade;
//3.和having关键字一起使用
select sum(grade),name from student group by grade having sum(grade) >100;
//使用limit限制查询结果的数量
select * from student limit 5;
select * from student limit 2,2;
select * from student order by grade desc limit 2,2;
//函数,mysql提供了许多函数
select concat(id,':',name,':',grade) from student;
//为表取别名
select * from student as stu where stu.name="howie";
//为字段取别名,as关键字也可以不写
select name as stu_name,grade stu_grade from student;

多表操作

1.了解外键 2.了解关联关系 3.了解各种连接查询多表的数据 4.了解子查询,会使用各种关键字以及比较运算符查询多表中的数据

外键 外键是指引用另一个表中的一列或者多列,被引用的列应该具有主键约束或者唯一性约束,用于建立和加强两个数据表之间的连接。

//创建表class,student
create table class(
   id int not null primary key,
   classname varchar(20) not null
)ENGINE=InnoDB;
create table student(
   stu_id int not null primary key,
   stu_name varchar(20) not null,
   cid int not null      -- 表示班级id,它就是class表的外键
)ENGINE=InnoDB;
//添加外键约束
alter table student add constraint FK_ID foreign key(cid) references class(id);
//删除外键约束
alter table student drop foreign key FK_ID;

操作关联表

//数据表有三种关联关系,多对一、多对多、一对一
//学生(student)和班级(class)是多对一关系,添加数据
//首选添加外键约束
alter table student add constraint FK_ID foreign key(cid) references class(id);
//添加数据,这两个表便有了关联若插入中文在终端显示空白,可设置set names 'gbk';
insert into class values(1,"软件一班"),(2,"软件二班");
insert into student values(1,"howie",1),(2,"howie1",2),(3,"howie2",1),(4,"howie3",2);
//交叉连接
select * from student cross join class;
//内连接,该功能也可以使用where语句实现
select student.stu_name,class.classname from student join class on class.id=student.cid;
//外连接
//首先在student,class表中插入数据
insert into class values(3,"软件三班");
//左连接,右连接
select s.stu_id,s.stu_name,c.classname from student s left join class c on c.id=s.cid;
select s.stu_id,s.stu_name,c.classname from student s right join class c on c.id=s.cid;
//复合条件连接查询就是添加过滤条件
//子查询
//in关键字子查询跟上面的in关键字查询类似
select * from student where cid in(select id from class where id=2);
//exists关键字查询,相当于测试,不产生数据,只返回true或者false,只有返回true,外层才会执行,具体看下图
select * from student where exists(select id from class where id=12);   -- 外层不会执行
select * from student where exists(select id from class where id=1);    -- 外层会执行
//any关键字查询
 select * from student where cid>any(select id from class);
//all关键字查询
 select * from student where cid=any(select id from class);