mysql统计结果大于0时返回true_mysql表查询练习-白红宇的个人博客

mysql统计结果大于0时返回true_mysql表查询练习

发布日期：2021-06-24 11:31:06 浏览次数：3 分类：技术文章

本文共 17206 字，大约阅读时间需要 57 分钟。

准备数据

create table book(

id int primary key auto_increment,

name varchar(16),

price int

);

create table author(

id int primary key auto_increment,

name varchar(16)

);

create table book_author(

id int primary key auto_increment,

book_id int,

author_id int

);

name

pirce

葵花宝典

九阴真经

辟邪剑谱

name

张全蛋

李诞

寡人

book_id

author_id

2、查看每本书的作者

SELECT

book.NAME,

GROUP_CONCAT( author.`name` )

FROM

book

INNER JOIN book_to_author ON book.id = book_to_author.book_id

INNER JOIN author ON author.id = author_id

GROUP BY

book.NAME;

3、查看每位作者都有哪些作品

SELECT

author.NAME,

group_concat( book.NAME )

FROM

author

INNER JOIN book_to_author ON author.id = book_to_author.author_id

INNER JOIN book ON book.id = book_to_author.book_id

GROUP BY

author.NAME;

4、利用pymysql操作数据库实现sql注入效果

# -*- coding: utf-8 -*-

import pymysql

# 实例化产生一个 Connect 连接对象

conn = pymysql.Connect(

host='127.0.0.1',

port=3306,

user='root',

password='root',

database='day41',

charset='utf8'

)

# 游标对象

name = input('>>>name:').strip()

pwd = input('>>>>pwd:').strip()

cursor = conn.cursor(pymysql.cursors.DictCursor)

sql = "select * from userinfo where name='%s' and password=%s" % (name, pwd)

print(sql)

res = cursor.execute(sql)

print(res)

if res:

print('登录成功')

else:

print('登录失败')

# >>>name:qzk' -- dasad

# >>>>pwd:

# select * from userinfo where name='qzk' -- dasad' and password=

# 1

# 登录成功

# >>>name:qzk' or 1=1 -- asd

# >>>>pwd:''

# select * from userinfo where name='qzk' or 1=1 -- asd' and password=''

# 5

# 登录成功

5、利用pymysql实现登陆注册功能

登陆:展示对应的错误信息(用户名错误，密码错误)

# 实例化产生一个 Connect 连接对象

conn = pymysql.Connect(

host='127.0.0.1',

port=3306,

user='root',

password='root',

database='day41',

charset='utf8'

)

# 游标对象

name = input('>>>name:').strip()

pwd = input('>>>>pwd:').strip()

cursor = conn.cursor(pymysql.cursors.DictCursor)

sql = 'select * from userinfo where name=%s'

print(sql)

res = cursor.execute(sql,(name,))

print(res)

dic_user = cursor.fetchone()

print(dic_user)

if res:

if int(pwd) == dic_user['password']:

print('登录成功')

else:

print('密码错误')

else:

print('用户名不存在')

内容回顾

select distinct 字段1，字段2，。。。 from 表名

where分组之前的过滤条件

group by 分组条件

having分组之后过滤条件

order by 排序字段1 asc，排序字段2 desc

limit 5,5

as语法中给某个查询结果起别名的时候需要把查询语句中的分号去除

(select name,salary*12 as '年薪' from emp) as t1;

# 一个字段展示用户名和年龄

select concat(name,':',age) as info from emp;

# 字段为NAME和AGE，值为‘NAME:jason’,'AGE:18'

select concat("NAME:",name) as NAME,concat("AGE:",age) as AGE from emp;

# 如果拼接的符号是统一的可以用

select concat_ws(':',name,age,sex) as info from emp;

# 1.子查询相关

# 查询平均年轻在25岁以上的部门名

select name from dep

where id in

(select dep_id from emp group by dep_id having avg(age)>25);

select dep.name from emp inner join dep on emp.dep_id = dep.id

group by dep.name

having avg(age) > 25;

# exist(了解)

EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录，

而是返回一个真假值，True或False。

当返回True时，外层查询语句将进行查询

当返回值为False时，外层查询语句不进行查询。

select * from employee

where exists

(select id from department where id > 3);

select * from employee

where exists

(select id from department where id > 250);

Navicat使用

掌握：

#1. 测试+链接数据库

#2. 新建库

#3. 新建表，新增字段+类型+约束

#4. 设计表：外键

#5. 新建查询

#6. 建立表模型

#注意：

批量加注释：ctrl+？键

批量去注释：ctrl+shift+？键

练习题

导出的sql语句代码

数据导入：

Navicat Premium Data Transfer

Source Server : localhost

Source Server Type : MySQL

Source Server Version : 50624

Source Host : localhost

Source Database : sqlexam

Target Server Type : MySQL

Target Server Version : 50624

File Encoding : utf-8

Date: 10/21/2016 06:46:46 AM

SET NAMES utf8;

SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------

-- Table structure for `class`

-- ----------------------------

DROP TABLE IF EXISTS `class`;

CREATE TABLE `class` (

`cid` int(11) NOT NULL AUTO_INCREMENT,

`caption` varchar(32) NOT NULL,

PRIMARY KEY (`cid`)

) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of `class`

-- ----------------------------

BEGIN;

INSERT INTO `class` VALUES ('1', '三年二班'), ('2', '三年三班'), ('3', '一年二班'), ('4', '二年九班');

COMMIT;

-- ----------------------------

-- Table structure for `course`

-- ----------------------------

DROP TABLE IF EXISTS `course`;

CREATE TABLE `course` (

`cid` int(11) NOT NULL AUTO_INCREMENT,

`cname` varchar(32) NOT NULL,

`teacher_id` int(11) NOT NULL,

PRIMARY KEY (`cid`),

KEY `fk_course_teacher` (`teacher_id`),

CONSTRAINT `fk_course_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`tid`)

) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of `course`

-- ----------------------------

BEGIN;

INSERT INTO `course` VALUES ('1', '生物', '1'), ('2', '物理', '2'), ('3', '体育', '3'), ('4', '美术', '2');

COMMIT;

-- ----------------------------

-- Table structure for `score`

-- ----------------------------

DROP TABLE IF EXISTS `score`;

CREATE TABLE `score` (

`sid` int(11) NOT NULL AUTO_INCREMENT,

`student_id` int(11) NOT NULL,

`course_id` int(11) NOT NULL,

`num` int(11) NOT NULL,

PRIMARY KEY (`sid`),

KEY `fk_score_student` (`student_id`),

KEY `fk_score_course` (`course_id`),

CONSTRAINT `fk_score_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`cid`),

CONSTRAINT `fk_score_student` FOREIGN KEY (`student_id`) REFERENCES `student` (`sid`)

) ENGINE=InnoDB AUTO_INCREMENT=53 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of `score`

-- ----------------------------

BEGIN;

INSERT INTO `score` VALUES ('1', '1', '1', '10'), ('2', '1', '2', '9'), ('5', '1', '4', '66'), ('6', '2', '1', '8'), ('8', '2', '3', '68'), ('9', '2', '4', '99'), ('10', '3', '1', '77'), ('11', '3', '2', '66'), ('12', '3', '3', '87'), ('13', '3', '4', '99'), ('14', '4', '1', '79'), ('15', '4', '2', '11'), ('16', '4', '3', '67'), ('17', '4', '4', '100'), ('18', '5', '1', '79'), ('19', '5', '2', '11'), ('20', '5', '3', '67'), ('21', '5', '4', '100'), ('22', '6', '1', '9'), ('23', '6', '2', '100'), ('24', '6', '3', '67'), ('25', '6', '4', '100'), ('26', '7', '1', '9'), ('27', '7', '2', '100'), ('28', '7', '3', '67'), ('29', '7', '4', '88'), ('30', '8', '1', '9'), ('31', '8', '2', '100'), ('32', '8', '3', '67'), ('33', '8', '4', '88'), ('34', '9', '1', '91'), ('35', '9', '2', '88'), ('36', '9', '3', '67'), ('37', '9', '4', '22'), ('38', '10', '1', '90'), ('39', '10', '2', '77'), ('40', '10', '3', '43'), ('41', '10', '4', '87'), ('42', '11', '1', '90'), ('43', '11', '2', '77'), ('44', '11', '3', '43'), ('45', '11', '4', '87'), ('46', '12', '1', '90'), ('47', '12', '2', '77'), ('48', '12', '3', '43'), ('49', '12', '4', '87'), ('52', '13', '3', '87');

COMMIT;

-- ----------------------------

-- Table structure for `student`

-- ----------------------------

DROP TABLE IF EXISTS `student`;

CREATE TABLE `student` (

`sid` int(11) NOT NULL AUTO_INCREMENT,

`gender` char(1) NOT NULL,

`class_id` int(11) NOT NULL,

`sname` varchar(32) NOT NULL,

PRIMARY KEY (`sid`),

KEY `fk_class` (`class_id`),

CONSTRAINT `fk_class` FOREIGN KEY (`class_id`) REFERENCES `class` (`cid`)

) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of `student`

-- ----------------------------

BEGIN;

INSERT INTO `student` VALUES ('1', '男', '1', '理解'), ('2', '女', '1', '钢蛋'), ('3', '男', '1', '张三'), ('4', '男', '1', '张一'), ('5', '女', '1', '张二'), ('6', '男', '1', '张四'), ('7', '女', '2', '铁锤'), ('8', '男', '2', '李三'), ('9', '男', '2', '李一'), ('10', '女', '2', '李二'), ('11', '男', '2', '李四'), ('12', '女', '3', '如花'), ('13', '男', '3', '刘三'), ('14', '男', '3', '刘一'), ('15', '女', '3', '刘二'), ('16', '男', '3', '刘四');

COMMIT;

-- ----------------------------

-- Table structure for `teacher`

-- ----------------------------

DROP TABLE IF EXISTS `teacher`;

CREATE TABLE `teacher` (

`tid` int(11) NOT NULL AUTO_INCREMENT,

`tname` varchar(32) NOT NULL,

PRIMARY KEY (`tid`)

) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

-- ----------------------------

-- Records of `teacher`

-- ----------------------------

BEGIN;

INSERT INTO `teacher` VALUES ('1', '张磊老师'), ('2', '李平老师'), ('3', '刘海燕老师'), ('4', '朱云海老师'), ('5', '李杰老师');

COMMIT;

SET FOREIGN_KEY_CHECKS = 1;

拷贝上述代码，新建一个.sql文件，保存到桌面

打开navicat新建数据库day41，选中新建的数据库鼠标右键选择运行SQL文件

弹出文件框，选中刚刚保存到桌面的.sql文件即可

快速建表

#准备表、记录 >>> 命令行

mysql> create database db1;

mysql> use db1;

mysql> source /root/init.sql

# navicat建表

练习题

1、查询所有的课程的名称以及对应的任课老师姓名

4、查询平均成绩大于八十分的同学的姓名和平均成绩

7、查询没有报李平老师课的学生姓名

8、查询没有同时选修物理课程和体育课程的学生姓名

9、查询挂科超过两门(包括两门)的学生姓名和班级

# 参考答案

#1、查询所有的课程的名称以及对应的任课老师姓名

SELECT

course.cname,

teacher.tname

FROM

course

INNER JOIN teacher ON course.teacher_id = teacher.tid;

#4、查询平均成绩大于八十分的同学的姓名和平均成绩

SELECT

student.sname,

t1.avg_num

FROM

student

INNER JOIN (

SELECT

student_id,

avg(num) AS avg_num

FROM

score

GROUP BY

student_id

HAVING

avg(num) > 80

) AS t1 ON student.sid = t1.student_id;

#7、查询没有报李平老师课的学生姓名(找出报名李平老师课程的学生，然后取反就可以)

SELECT

student.sname

FROM

student

WHERE

sid NOT IN (

SELECT DISTINCT

student_id

FROM

score

WHERE

course_id IN (

SELECT

course.cid

FROM

course

INNER JOIN teacher ON course.teacher_id = teacher.tid

WHERE

teacher.tname = '李平老师'

)

);

#8、查询没有同时选修物理课程和体育课程的学生姓名(没有同时选修指的是选修了一门的，思路是得到物理+体育课程的学生信息表，然后基于学生分组，统计count(课程)=1)

SELECT

student.sname

FROM

student

WHERE

sid IN (

SELECT

student_id

FROM

score

WHERE

course_id IN (

SELECT

cid

FROM

course

WHERE

cname = '物理'

OR cname = '体育'

)

GROUP BY

student_id

HAVING

COUNT(course_id) = 1

);

# 9、查询挂科超过两门(包括两门)的学生姓名和班级

select student.sname,class.caption from class INNER JOIN student

on class.cid = student.class_id

WHERE student.sid in

(select student_id from score where num < 60

GROUP BY student_id

HAVING COUNT(course_id) >=2)

;

pymysql模块

# 1.安装:pip3 insatll pymysql

# 2.代码链接

import pymysql

#链接

conn=pymysql.connect(

host='localhost',

user='root',

password='123',

database='egon',

charset='utf8')

#游标

cursor=conn.cursor() #执行完毕返回的结果集默认以元组显示

#cursor=conn.cursor(cursor=pymysql.cursors.DictCursor) # 以字典的方式显示数据

# 3.pymysql操作数据库

#执行sql语句

user = input(">>>:").strip()

pwd = input(">>>:").strip()

sql='select * from userinfo where name="%s" and password="%s"' %(user,pwd) #注意%s需要加引号

rows=cursor.execute(sql) #执行sql语句，返回sql查询成功的记录数目

# 获取真实数据cursor.fetchone(),cursor.fetchall(),cursor.fetchmany(),类似管道取值，获取一条，所有，多条

cursor.scroll(1,'relative') # 相对移动

cursor.scroll(3,'absolute') # 绝对移动

cursor.close()

conn.close()

sql注入问题

# 不要手动去拼接查询的sql语句

username = input(">>>:").strip()

password = input(">>>:").strip()

sql = "select * from user where username='%s' and password='%s'"%(username,password)

# 用户名正确

username >>>: jason' -- jjsakfjjdkjjkjs

# 用户名密码都不对的情况

username >>>: xxx' or 1=1 --asdjkdklqwjdjkjasdljad

password >>>: ''

增删改

# 增

sql = "insert into user(username,password) values(%s,%s)"

rows = cursor.excute(sql,('jason','123'))

# 修改

sql = "update user set username='jasonDSB' where id=1"

rows = cursor.excute(sql)

"""

增和改单单执行excute并不会真正影响到数据，需要再执行conn.commit()才可以完成真正的增改

"""

# 一次插入多行记录

res = cursor,excutemany(sql,[(),(),()]

1、查询所有的课程的名称以及对应的任课老师姓名

SELECT

cname,

tname

FROM

course

INNER JOIN teacher

course.teacher_id = teacher.tid;

2、查询学生表中男女生各有多少人

SELECT

gender, count( gender )

FROM

student

GROUP BY

gender

HAVING

count( gender );

3、查询物理成绩等于100的学生的姓名

SELECT

student.sname,

course.cname,

num

FROM

student

INNER JOIN score ON student.sid = score.student_id

INNER JOIN course ON course_id = course.cid

WHERE

cname = '物理'

AND num = 100;

4、查询平均成绩大于八十分的同学的姓名和平均成绩

SELECT

STUDENT.sname,

avg( num )

FROM

student

INNER JOIN score ON student.sid = score.student_id

GROUP BY

student.sname

HAVING

avg( num ) > 80;

5、查询所有学生的学号，姓名，选课数，总成绩

SELECT

student.sid,

sname,

count( course_id ),

sum( num )

FROM

student

INNER JOIN score ON student.sid = score.student_id

INNER JOIN course ON score.course_id = course.cid

GROUP BY

sname

ORDER BY

student.sid;

6、查询姓李老师的个数

SELECT

count( tname )

FROM

teacher

WHERE

tname LIKE '%李%';

7、查询没有报李平老师课的学生姓名

SELECT

sname

FROM

student

WHERE

student.sid not IN ( SELECT student_id FROM score WHERE course_id IN ( SELECT cid FROM course INNER JOIN teacher ON teacher_id = tid WHERE tname like '%李平%' ) );

8、查询物理课程比生物课程高的学生的学号

# 方法一、

SELECT

t1.student_id

FROM

( SELECT student_id, num FROM score INNER JOIN course ON course_id = cid WHERE cname = '物理' ) AS t1

INNER JOIN ( SELECT student_id, num FROM score INNER JOIN course ON course_id = cid WHERE cname = '生物' ) AS t2 ON t1.student_id = t2.student_id

WHERE

t1.num > t2.num;

# 方法二、

SELECT

t1.student_id

FROM

(

SELECT

student_id,

num

FROM

score

WHERE

course_id = (

SELECT

cid

FROM

course

WHERE

cname = '物理'

)

) AS t1

INNER JOIN (

SELECT

student_id,

num

FROM

score

WHERE

course_id = (

SELECT

cid

FROM

course

WHERE

cname = '生物'

)

) AS t2 ON t1.student_id = t2.student_id

WHERE

t1.num > t2.num;

9、查询没有同时选修物理课程和体育课程的学生姓名

# 方法一、

SELECT

student.sname

FROM

student

INNER JOIN score ON student.sid = student_id

INNER JOIN course ON course_id = cid

WHERE

cname = '物理'

OR cname = '体育'

GROUP BY

student.sname

HAVING

count( sname ) = 1;

# 方法三、

SELECT

student.sname

FROM

student

INNER JOIN score ON student.sid = student_id

INNER JOIN course ON course_id = cid

WHERE

cname = '物理'

OR cname = '体育'

GROUP BY

student.sname

HAVING

count( course_id ) = 1;

# 方法二、

SELECT

student.sname

FROM

student

WHERE

sid IN (

SELECT

student_id

FROM

score

WHERE

course_id IN (

SELECT

cid

FROM

course

WHERE

cname = '物理'

OR cname = '体育'

)

GROUP BY

student_id

HAVING

COUNT(course_id) = 1

);

10、查询挂科超过两门(包括两门)的学生姓名和班级、查询选修了所有课程的学生姓名

# 方法一、

SELECT

student.sname,

class.caption

FROM

class

INNER JOIN student ON class.cid = student.class_id

INNER JOIN score ON score.student_id = student.sid

WHERE

num < 60

GROUP BY

student.sname

HAVING

count( course_id ) >= 2;

# 方法二、

SELECT

student.sname,

class.caption

FROM

student

INNER JOIN (

SELECT

student_id

FROM

score

WHERE

num < 60

GROUP BY

student_id

HAVING

count(course_id) >= 2

) AS t1

INNER JOIN class ON student.sid = t1.student_id

AND student.class_id = class.cid;

12、查询李平老师教的课程的所有成绩记录

SELECT

num

FROM

score

WHERE

course_id IN ( SELECT cid FROM course INNER JOIN teacher ON teacher.tid = course.teacher_id WHERE tname = '李平老师' );

13、查询全部学生都选修了的课程号和课程名(count(sid)=13

SELECT

cid,

cname

FROM

course

WHERE

cid IN (

SELECT

course_id

FROM

score

GROUP BY

course_id

HAVING

COUNT(student_id) = (

SELECT

COUNT(sid)

FROM

student

)

);

14、查询每门课程被选修的次数

SELECT

cid,cname,

count( student_id )

FROM

course

INNER JOIN score ON course.cid = score.course_id

GROUP BY

cid;

# 方法二

SELECT

course_id,

COUNT(student_id)

FROM

score

GROUP BY

course_id;

15、查询之选修了一门课程的学生姓名和学号

SELECT

student.sname,

student.sid

FROM

student

WHERE

student.sid IN ( SELECT student_id FROM score GROUP BY student_id HAVING count( course_id ) = 1 );

16、查询所有学生考出的成绩并按从高到低排序(成绩去重)

17、查询平均成绩大于85的学生姓名和平均成绩

SELECT

student.sname,

avg( num )

FROM

student

INNER JOIN score ON student.sid = score.student_id

GROUP BY

student.sname

HAVING

avg( num ) > 80;

18、查询生物成绩不及格的学生姓名和对应生物分数

SELECT

student.sname,

course.cname,

score.num

FROM

student

INNER JOIN score ON student.sid = student_id

INNER JOIN course ON course_id = course.cid

WHERE

course.cname = '生物'

AND num < 60;

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名

20、查询每门课程成绩最好的前两名学生姓名

#20、查询每门课程成绩最好的前两名学生姓名

#查看每门课程按照分数排序的信息，为下列查找正确与否提供依据

SELECT

FROM

score

ORDER BY

course_id,

num DESC;

#表1：求出每门课程的课程course_id,与最高分数first_num

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id;

#表2：去掉最高分，再按照课程分组，取得的最高分，就是第二高的分数second_num

SELECT

score.course_id,

max(num) second_num

FROM

score

INNER JOIN (

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t ON score.course_id = t.course_id

WHERE

score.num < t.first_num

GROUP BY

course_id;

#将表1和表2联合到一起，得到一张表t3，包含课程course_id与该们课程的first_num与second_num

SELECT

t1.course_id,

t1.first_num,

t2.second_num

FROM

(

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t1

INNER JOIN (

SELECT

score.course_id,

max(num) second_num

FROM

score

INNER JOIN (

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t ON score.course_id = t.course_id

WHERE

score.num < t.first_num

GROUP BY

course_id

) AS t2 ON t1.course_id = t2.course_id;

#查询前两名的学生(有可能出现并列第一或者并列第二的情况)

SELECT

score.student_id,

t3.course_id,

t3.first_num,

t3.second_num

FROM

score

INNER JOIN (

SELECT

t1.course_id,

t1.first_num,

t2.second_num

FROM

(

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t1

INNER JOIN (

SELECT

score.course_id,

max(num) second_num

FROM

score

INNER JOIN (

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t ON score.course_id = t.course_id

WHERE

score.num < t.first_num

GROUP BY

course_id

) AS t2 ON t1.course_id = t2.course_id

) AS t3 ON score.course_id = t3.course_id

WHERE

score.num >= t3.second_num

AND score.num <= t3.first_num;

#排序后可以看的明显点

SELECT

score.student_id,

t3.course_id,

t3.first_num,

t3.second_num

FROM

score

INNER JOIN (

SELECT

t1.course_id,

t1.first_num,

t2.second_num

FROM

(

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t1

INNER JOIN (

SELECT

score.course_id,

max(num) second_num

FROM

score

INNER JOIN (

SELECT

course_id,

max(num) first_num

FROM

score

GROUP BY

course_id

) AS t ON score.course_id = t.course_id

WHERE

score.num < t.first_num

GROUP BY

course_id

) AS t2 ON t1.course_id = t2.course_id

) AS t3 ON score.course_id = t3.course_id

WHERE

score.num >= t3.second_num

AND score.num <= t3.first_num

ORDER BY

course_id;

#可以用以下命令验证上述查询的正确性

SELECT

FROM

score

ORDER BY

course_id,

num DESC;

转载地址：https://blog.csdn.net/weixin_32512381/article/details/113219402 如侵犯您的版权，请留言回复原文章的地址，我们会给您删除此文章，给您带来不便请您谅解！

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