MySQL-06-DQL语句

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DQL

sql文件下载链接:
https://alnk-blog-pictures.oss-cn-shenzhen.aliyuncs.com/blog-pictures/world.sql

select

-- select @@xxx 查看系统参数
SELECT @@port;
SELECT @@basedir;
SELECT @@datadir;
SELECT @@socket;
SELECT @@server_id;

-- select 函数();
SELECT NOW();
SELECT DATABASE();
SELECT USER();
SELECT CONCAT("hello world");
SELECT CONCAT(USER,"@",HOST) FROM mysql.user;
SELECT GROUP_CONCAT(USER,"@",HOST) FROM mysql.user;

-- https://dev.mysql.com/doc/refman/5.7/en/func-op-summary-ref.html?tdsourcetag=s_pcqq_aiomsg

from

-- SELECT 列1,列2 FROM 表
-- SELECT  *  FROM 表

-- 例子:
-- 查询stu中所有的数据(不要对大表进行操作)
SELECT * FROM stu ;

-- 查询stu表中,学生姓名和入学时间
SELECT sname, intime FROM stu;

-- city:城市表
DESC city;
/**
ID :         城市ID
NAME :       城市名
CountryCode: 国家代码，比如中国CHN 美国USA
District :   区域
Population : 人口
**/

SHOW CREATE TABLE city;
SELECT * FROM city WHERE id<10;

where

-- SELECT col1,col2 FROM TABLE WHERE colN 条件;

-- 1 where配合等值查询
-- 查询中国(CHN)所有城市信息
SELECT * FROM city WHERE countrycode='CHN';
-- 查询北京市的信息
SELECT * FROM city WHERE NAME='peking';
-- 查询甘肃省所有城市信息
SELECT * FROM city WHERE district='gansu';

-- 2 where配合比较操作符(> < >= <= <>)
-- 查询世界上少于100人的城市
SELECT * FROM city WHERE population<100;

-- 3 where配合逻辑运算符(and or )
-- 中国人口数量大于500w
SELECT * FROM city WHERE countrycode='CHN' AND population>5000000;
-- 中国或美国城市信息
SELECT * FROM city WHERE countrycode='CHN' OR countrycode='USA';

-- 4 where配合模糊查询
-- 查询省的名字前面带guang开头的
SELECT * FROM city WHERE district LIKE 'guang%';
-- 注意:%不能放在前面,因为不走索引.

-- 5 where配合in语句
-- 中国或美国城市信息
SELECT * FROM city WHERE countrycode IN ('CHN' ,'USA');

-- 6 where配合between and
-- 查询世界上人口数量大于100w小于200w的城市信息
SELECT * FROM city  WHERE population >1000000 AND population <2000000;
SELECT * FROM city  WHERE population BETWEEN 1000000 AND 2000000;

group by

-- 1 作用
-- 根据by后面的条件进行分组，方便统计，by后面跟一个列或多个列

/**
2 常用聚合函数
max()      ：最大值
min()      ：最小值
avg()      ：平均值
sum()      ：总和
count()    ：个数
group_concat() : 列转行
**/

-- 例子1：统计世界上每个国家的总人口数.
USE world
SELECT countrycode, SUM(population) FROM  city  GROUP BY countrycode;

-- 例子2： 统计中国各个省的总人口数量
SELECT district, SUM(Population) FROM city  WHERE countrycode='chn' GROUP BY district;

-- 例子3：统计世界上每个国家的城市数量
SELECT countrycode, COUNT(id) FROM city GROUP BY countrycode;

having

-- where|group|having

-- 统计中国每个省的总人口数，只打印总人口数小于100万
SELECT district,SUM(Population)
FROM city
WHERE countrycode='chn'
GROUP BY district
HAVING SUM(Population) < 1000000 ;

order by和limit

-- 实现先排序，by后添加条件列

-- 查看中国所有的城市，并按人口数进行排序(从大到小)
SELECT * FROM city WHERE countrycode='CHN' ORDER BY population DESC;

-- 统计中国各个省的总人口数量，按照总人口从大到小排序
SELECT district AS 省 ,SUM(Population) AS 总人口
FROM city
WHERE countrycode='chn'
GROUP BY district
ORDER BY 总人口 DESC ;

-- 统计中国,每个省的总人口,找出总人口大于500w的,并按总人口从大到小排序,只显示前三名
SELECT  district, SUM(population)  FROM  city
WHERE countrycode='CHN'
GROUP BY district
HAVING SUM(population)>5000000
ORDER BY SUM(population) DESC
LIMIT 3 ;

-- LIMIT N ,M --->跳过N,显示一共M行
-- LIMIT 5,5

SELECT district, SUM(population)  FROM  city
WHERE countrycode='CHN'
GROUP BY district
HAVING SUM(population)>5000000
ORDER BY SUM(population) DESC
LIMIT 5,5;

distinct

-- distinct:去重复
SELECT countrycode FROM city;
SELECT DISTINCT(countrycode) FROM city;

union all

-- union all:联合查询

-- 中国或美国城市信息
SELECT * FROM city
WHERE countrycode IN ('CHN' ,'USA');

SELECT * FROM city WHERE countrycode='CHN'
UNION ALL
SELECT * FROM city WHERE countrycode='USA'

/**
说明:一般情况下,我们会将 IN 或者 OR 语句 改写成 UNION ALL,来提高性能
UNION     去重复
UNION ALL 不去重复
**/

join

-- join:多表连接查询

-- 查询一下世界上人口数量小于100人的城市名和国家名
SELECT b.name ,a.name ,a.population
FROM  city  AS a
JOIN  country AS b
ON    b.code=a.countrycode
WHERE  a.Population<100

-- 查询城市shenyang，城市人口，所在国家名（name）及国土面积（SurfaceArea）
SELECT a.name,a.population,b.name ,b.SurfaceArea
FROM city  AS a JOIN country AS b
ON a.countrycode=b.code
WHERE a.name='shenyang';

-- 列别名,表别名
SELECT
a.Name AS an ,
b.name AS bn ,
b.SurfaceArea AS bs,
a.Population AS bp
FROM city AS a  JOIN country AS b
ON a.CountryCode=b.Code
WHERE a.name ='shenyang';

多表SQL练习题

/**
案例准备
按需求创建一下表结构:
use school
student：学生表
  sno：  学号
  sname：学生姓名
  sage： 学生年龄
  ssex： 学生性别

teacher：教师表
  tno：  教师编号
  tname：教师名字

course： 课程表
  cno：  课程编号
  cname：课程名字
  tno：  教师编号

score：  成绩表
  sno：  学号
  cno：  课程编号
  score：成绩
**/

-- 项目构建
drop database school;
CREATE DATABASE school CHARSET utf8;
USE school

CREATE TABLE student(
sno INT NOT NULL PRIMARY KEY AUTO_INCREMENT COMMENT '学号',
sname VARCHAR(20) NOT NULL COMMENT '姓名',
sage TINYINT UNSIGNED  NOT NULL COMMENT '年龄',
ssex  ENUM('f','m') NOT NULL DEFAULT 'm' COMMENT '性别'
)ENGINE=INNODB CHARSET=utf8;

CREATE TABLE course(
cno INT NOT NULL PRIMARY KEY COMMENT '课程编号',
cname VARCHAR(20) NOT NULL COMMENT '课程名字',
tno INT NOT NULL  COMMENT '教师编号'
)ENGINE=INNODB CHARSET utf8;

CREATE TABLE sc (
sno INT NOT NULL COMMENT '学号',
cno INT NOT NULL COMMENT '课程编号',
score INT  NOT NULL DEFAULT 0 COMMENT '成绩'
)ENGINE=INNODB CHARSET=utf8;

CREATE TABLE teacher(
tno INT NOT NULL PRIMARY KEY COMMENT '教师编号',
tname VARCHAR(20) NOT NULL COMMENT '教师名字'
)ENGINE=INNODB CHARSET utf8;

INSERT INTO student(sno,sname,sage,ssex)
VALUES (1,'zhang3',18,'m');

INSERT INTO student(sno,sname,sage,ssex)
VALUES
(2,'zhang4',18,'m'),
(3,'li4',18,'m'),
(4,'wang5',19,'f');

INSERT INTO student
VALUES
(5,'zh4',18,'m'),
(6,'zhao4',18,'m'),
(7,'ma6',19,'f');

INSERT INTO student(sname,sage,ssex)
VALUES
('oldboy',20,'m'),
('oldgirl',20,'f'),
('oldp',25,'m');

INSERT INTO teacher(tno,tname) VALUES
(101,'oldboy'),
(102,'hesw'),
(103,'oldguo');

DESC course;
INSERT INTO course(cno,cname,tno)
VALUES
(1001,'linux',101),
(1002,'python',102),
(1003,'mysql',103);

DESC sc;
INSERT INTO sc(sno,cno,score)
VALUES
(1,1001,80),
(1,1002,59),
(2,1002,90),
(2,1003,100),
(3,1001,99),
(3,1003,40),
(4,1001,79),
(4,1002,61),
(4,1003,99),
(5,1003,40),
(6,1001,89),
(6,1003,77),
(7,1001,67),
(7,1003,82),
(8,1001,70),
(9,1003,80),
(10,1003,96);

SELECT * FROM student;
SELECT * FROM teacher;
SELECT * FROM course;
SELECT * FROM sc;

-- 查询张三的家庭住址
SELECT A.name,B.address FROM
A JOIN  B
ON A.id=B.id
WHERE A.name='zhangsan'

-- 统计zhang3,学习了几门课
SELECT st.sname , COUNT(sc.cno)
FROM student AS st
JOIN
sc
ON st.sno=sc.sno
WHERE st.sname='zhang3'

-- 查询zhang3,学习的课程名称有哪些
SELECT st.sname , GROUP_CONCAT(co.cname)
FROM student AS st
JOIN sc
ON st.sno=sc.sno
JOIN course AS co
ON sc.cno=co.cno
WHERE st.sname='zhang3'

-- 查询oldguo老师教的学生名.
SELECT te.tname ,GROUP_CONCAT(st.sname)
FROM student AS st
JOIN sc
ON st.sno=sc.sno
JOIN course AS co
ON sc.cno=co.cno
JOIN teacher AS te
ON co.tno=te.tno
WHERE te.tname='oldguo';

select teacher.tname,GROUP_CONCAT(student.sname) from teacher
JOIN course on teacher.tno=course.tno
JOIN sc on course.cno=sc.cno
JOIN student on sc.sno=student.sno
where teacher.tname='oldguo'

-- 查询oldguo所教课程的平均分数
SELECT te.tname,AVG(sc.score)
FROM teacher AS te
JOIN course AS co
ON te.tno=co.tno
JOIN sc
ON co.cno=sc.cno
WHERE te.tname='oldguo'

-- 每位老师所教课程的平均分,并按平均分排序
SELECT te.tname,AVG(sc.score)
FROM teacher AS te
JOIN course AS co
ON te.tno=co.tno
JOIN sc
ON co.cno=sc.cno
GROUP BY te.tname
ORDER BY AVG(sc.score) DESC ;

-- 查询oldguo所教的不及格的学生姓名
SELECT te.tname,st.sname,sc.score
FROM teacher AS te
JOIN course  AS co
ON te.tno=co.tno
JOIN sc
ON co.cno=sc.cno
JOIN student AS st
ON sc.sno=st.sno
WHERE te.tname='oldguo' AND sc.score<60;

-- 查询所有老师所教学生不及格的信息
SELECT te.tname,st.sname,sc.score
FROM teacher AS te
JOIN course  AS co
ON te.tno=co.tno
JOIN sc
ON co.cno=sc.cno
JOIN student AS st
ON sc.sno=st.sno
WHERE sc.score<60;