MySQL

MySQL 是最流行的关系型数据库管理系统，在 WEB 应用方面 MySQL 是最好的 RDBMS(Relational Database Management System：关系数据库管理系统)应用软件之一。

MySQL查询

查询所有字段

select * from 表名;
例：
select * from students;

查询指定字段

select 列1,列2,... from 表名;
例:
select name from students;

使用 as 给字段起别名

select id as 序号, name as 名字, gender as 性别 from students;

例:
-- 如果是单表查询 可以省略表名
select id, name, gender from students;

-- 表名.字段名
select students.id,students.name,students.gender from students;

-- 可以通过 as 给表起别名 
select s.id,s.name,s.gender from students as s;

在select后面列前使用distinct可以消除重复的行

select distinct 列1,... from 表名;
例：
select distinct gender from students;

条件查询

使用where子句对表中的数据筛选，结果为true的行会出现在结果集中

select * from 表名 where 条件;
例：
select * from students where id=1;

where后面支持多种运算符，进行条件的处理

比较运算符
逻辑运算符
模糊查询
范围查询
空判断

比较运算符

等于: =
大于: >
大于等于: >=
小于: <
小于等于: <=
不等于: != 或 <>

例：查询编号大于3的学生
select * from students where id > 3;

例：查询编号不大于4的学生
select * from students where id <= 4;

例：查询姓名不是“黄蓉”的学生
select * from students where name != '黄蓉';

例：查询没被删除的学生
select * from students where is_delete=0;

逻辑运算符

例：查询编号大于3的女同学
select * from students where id > 3 and gender=0;

例：查询编号小于4或没被删除的学生
select * from students where id < 4 or is_delete=0;

模糊查询

like
%表示任意多个任意字符
_表示一个任意字符

例：查询姓黄的学生
select * from students where name like '黄%';
例：查询姓黄并且“名”是一个字的学生

select * from students where name like '黄_';
例：查询姓黄或叫靖的学生
select * from students where name like '黄%' or name like '%靖';

范围查询

in表示在一个非连续的范围内
between … and …表示在一个连续的范围内

例：查询编号是1或3或8的学生
select * from students where id in(1,3,8);

例：查询编号为3至8的学生
select * from students where id between 3 and 8;

例：查询编号是3至8的男生
select * from students where (id between 3 and 8) and gender=1;

空判断

is null 空
is not null 非空

例：查询没有填写身高的学生
select * from students where height is null;
判非空is not null

例：查询填写了身高的学生
select * from students where height is not null;

例：查询填写了身高的男生
select * from students where height is not null and gender=1;

优先级

优先级由高到低的顺序为：小括号，not，比较运算符，逻辑运算符
and比or先运算，如果同时出现并希望先算or，需要结合()使用

排序

语法：

select * from 表名 order by 列1 asc|desc [,列2 asc|desc,...]

注意

默认按照列值从小到大排列（asc）
asc从小到大排列，即升序
desc从大到小排序，即降序
将行数据按照列1进行排序，如果某些行列1的值相同时，则按照列2排序，以此类推

例：查询未删除男生信息，按学号降序
select * from students where gender=1 and is_delete=0 order by id desc;

例：查询未删除学生信息，按名称升序
select * from students where is_delete=0 order by name;

例：显示所有的学生信息，先按照年龄从大-->小排序，当年龄相同时 按照身高从高-->矮排序
select * from students  order by age desc,height desc;

聚合函数

总数

count(*)表示计算总行数，括号中写星与列名，结果是相同的

例：查询学生总数
select count(*) from students;

最大值

max(列)表示求此列的最大值

例：查询女生的编号最大值
select max(id) from students where gender=2;

最小值

min(列)表示求此列的最小值

例：查询未删除的学生最小编号
select min(id) from students where is_delete=0;

求和

sum(列)表示求此列的和

例：查询男生的总年龄
select sum(age) from students where gender=1;

-- 平均年龄
select sum(age)/count(*) from students where gender=1;

平均值

avg(列)表示求此列的平均值

例：查询未删除女生的编号平均值
select avg(id) from students where is_delete=0 and gender=2;

分组

group by

group by的含义:将查询结果按照1个或多个字段进行分组，字段值相同的为一组
group by可用于单个字段分组，也可用于多个字段分组

group by + group_concat()

例：统计学生的性别和每个性别包含的学生名字
select gender,group_concat(name) from students group by gender;

group by + 集合函数

例：分别统计每个性别的人年龄平均值
select gender,avg(age) from students group by gender;

例：分别统计每个性别的人的个数
select gender,count(*) from students group by gender;

group by + having

having 条件表达式：用来分组查询后指定一些条件来输出查询结果
having作用和where一样，但having只能用于group by

例：
select gender,count(*) from students group by gender having count(*)>2;

group by + with rollup

with rollup的作用是：在最后新增一行，来记录当前列里所有记录的总和

例：
select gender,count(*) from students group by gender with rollup;

分页

语法：

从start开始，每页获取count条数据
select * from 表名 limit start,count

例：查询前3行男生信息
select * from students where gender=1 limit 0,3;

每页显示m条数据，当前显示第n页
求第n页的数据
select * from students where is_delete=0 limit (n-1)*m,m

MySQL操作

6.3MySQL操作1 6.3MySQL操作2 6.3MySQL操作3 6.3MySQL操作4

具体操作：

CREATE DATABASE 07test charset=utf8;

use 07test

-- 创建学生表
create table student(
	sno varchar(20) primary key,
	sname  varchar(10) not null,
	ssex varchar(10) not null,
	sbirthday datetime,
	class varchar(20)
);

-- 创建教师表
create table teacher(
	tno varchar(20) primary key,
	tname varchar(20) not null,
	tsex varchar(20) not null,
	tbirthday datetime,
	pro varchar(20) not null,
	depart varchar(20) not null
);

-- 创建课程表
create table course(
  cno varchar(20) primary key,	
	cname varchar(20) not null,
	tno varchar(20) not null,
	foreign key(tno) references teacher(tno)
);

-- 创建成绩表
create table score(
	sno varchar(20) not null,
	cno varchar(20) not null,
	degree decimal(10, 0),
	foreign key(sno) references student(sno),
	foreign key(cno) references course(cno),
	primary key(sno, cno)
);


-- 1.查询student表中的所有记录
select * from student;


-- 2.检查student表中的所有记录的sname，ssex和class列。
select sname, ssex, class from student;


-- 3.查询老师所有的单位即不重复的depart distinct
select distinct depart from teacher;


-- 4.查询socre表中成绩在从68到80之间的记录。
select * from score where degree between 68 and 80;


-- 5.查询score表成绩中为85、86或88。
select * from score where degree in(85, 86, 88);


-- 6.查询student表中“95031”班或性别为“女”同学的同学记录
select * from student where class='95031' or ssex='女';


-- 7.以class降序查询student表的所有记录
select * from student order by class desc;


-- 8.以cno升序，degree降序查询score表的所有记录
select * from score order by cno asc, degree desc;


-- 9.查询“95031”班学生人数。
select count(*) from student where class='95031';


-- 10.查询score表中的最高分的学生学号和课程号（子查询或者排序）
select max(degree) from score;

select sno,cno from score where degree=(select max(degree) from score);

select sno,cno,degree from score order by degree desc limit 0,4;


-- 11.查看每门课的平均成绩 (分组查询)
select cno, avg(degree) from score group by cno;


-- 12.查询score表中至少有2名同学选修的并以3开头的课程的平均分数 havimg(组级过滤)
-- having 条件表达式：用来分组查询后指定一些条件来输出查询结果
-- having作用和where一样，但having只能用于group by
1.将成绩表按课程分组
select * from score group by cno;
2.将分组后的结果按至少2名同学分
select * from score group by cno having count(cno) >= 2;
3.将课程名以3开头
select * from score where cno like '3%' group by cno having count(cno) >= 2;


select cno,avg(degree), count(*) from score group by cno having count(cno) >= 2 and cno like '3%';


-- 13.查询分数大于70，小于90的sno列（条件查询）
select sno, degree from score where degree between 70 and 90;

select sno, degree from score where degree>70 and degree<90;


-- 14.查询所有学生的 sname,cno,和degree 列。（多表查询）
select sno, sname from student;

select cno, degree from score;

select sname, cno, degree from student, score where student.sno = score.sno;


--  15.查询所有学生的sno，cname和degree
select cno, cname from course;

select sno, cno, degree from score;

select sno, cname, degree from course, score where course.cno = score.cno;

-- 16.查询所有学生的sname，cname和degree(三表联查)
select sname, cname, degree from student, course, score where student.sno = score.sno and course.cno = score.cno;


-- 17.查阅“95031"班学生每门课的平均分
select * from student where class='95031';

select * from score where sno in (select sno from student where class='95031');

select sno, avg(degree) from score where sno in (select sno from student where class='95031') group by cno;


-- 18.查询选修”3-105“课程的成绩高于”109“号同学”3-105“成绩的所有同学的记录
1.先查询”109“号同学”3-105“成绩
select degree from score where sno='109' and cno='3-105';
2.再找到选修”3-105“课程的成绩高于”109“号同学”3-105“成绩
select * from score where cno='3-105' and degree>(select degree from score where sno='109' and cno='3-105');

-- 19.查询成绩高于学号109，课程号为3-105的成绩的所有记录 子查询
1.先查询”109“号同学”3-105“成绩
select degree from score where sno='109' and cno='3-105';

2.再查询高于他成绩的
select * from score where degree>(select degree from score where sno='109' and cno='3-105');



-- 20.查询和学号为108.101的同学同年出生的所有学生的sno。sname和sbirthday的函数与带in 关键字的子查询
1.先查询和学号为108.101的同学的生日
select year(sbirthday) from student where sno in (108,101);

select sno, sname, sbirthday from student where year(sbirthday) in (select year(sbirthday) from student where sno in (108,101));

-- 21.查询”张旭“教师任课的学生成绩
1.先查询教师的id号
select tno from teacher where tname="张旭";

2.然后根据教师的id号来查询课程id
select cno from course where tno=(select tno from teacher where tname="张旭"); 

3.查询”张旭“教师任课的学生成绩
select degree from score where cno = (select cno from course where tno=(select tno from teacher where tname="张旭"));

MySQL进阶