SQL基础应用（二）

DQL应用 select

select单独使用的情况

-- select @@xxx 查看系统参数
SELECT @@port;
SELECT @@basedir;
SELECT @@datadir;
SELECT @@socket;
SELECT @@server_id;
select @@innodb_flush_log_at_trx_commit;
show variables like 'innodb%';

SQL基础应用（二）

mysql函数参考手册

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

mysql 书写顺序

select  列   
from  表   
where 条件  
group by  条件 
having   条件 
order by 条件
limit

练习表说明

city 城市表
country 国家表
countrylanguage 国家的语言

city表结构

mysql> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   |     |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+

5 rows in set (0.00 sec)
mysql> 

ID  		:  	城市序号(1-...)
name		: 	城市名字
countrycode :   国家代码,例如:CHN,USA
district    :   区域: 中国 省  美国 洲
population  :   人口数

快速了解表结构

DESC ,show create table
select * from city limit 5;

SELECT 配合 FROM 子句使用

查询表中所有的信息

SELECT  id,NAME ,countrycode ,district,population   FROM  city;
或者:
SELECT  *   FROM city;

查询表中 name和population的值

SELECT  NAME ,population   FROM  city;

SELECT 配合 WHERE 子句使用

查询中国所有的城市名和人口数

SELECT  NAME,population FROM city WHERE countrycode='CHN';

世界上小于100人的城市名和人口数

SELECT NAME,population FROM city WHERE population<100;

查询中国人口数量大于1000w的城市名和人口

SELECT NAME,population FROM city WHERE countrycode='CHN' AND population>8000000;

查询中国或美国的城市名和人口数

SELECT NAME,population FROM city WHERE countrycode='CHN' OR countrycode='USA';

查询人口数量在500w到600w之间的城市名和人口数

SELECT NAME,population FROM city WHERE population>5000000 AND population<6000000;
或者:
SELECT NAME,population FROM city WHERE population BETWEEN 5000000 AND 6000000;

查询一下contrycode中带有CH开头,城市信息

SELECT * FROM city  WHERE countrycode  LIKE 'CH%';

注意:不要出现类似于 %CH%,前后都有百分号的语句,因为不走索引,性能极差如果业务中有大量需求,我们用"ES"来替代

查询中国或美国的城市信息

SELECT NAME,population FROM city WHERE countrycode='CHN' OR countrycode='USA';
或者:
SELECT NAME,population FROM city WHERE countrycode IN ('CHN' ,'USA');

GROUP BY

用途

将某列中有共同条件的数据行,分成一组,然后在进行聚合函数操作

统计每个国家,城市的个数

SELECT countrycode ,COUNT(id) FROM city GROUP BY countrycode;

统计每个国家的总人口数

SELECT countrycode,SUM(population) FROM city GROUP BY countrycode;

统计每个国家省的个数

SELECT countrycode, COUNT(DISTINCT district)  FROM city GROUP BY countrycode;

统计中国每个省的总人口数

SELECT district, SUM(population) FROM  city WHERE countrycode='CHN' GROUP BY district;

统计中国每个省城市的个数

SELECT district, COUNT(NAME) FROM city WHERE countrycode='CHN' GROUP BY district;

统计中国每个省城市的名字列表GROUP_CONCAT()

SELECT district, GROUP_CONCAT(NAME) FROM  city WHERE countrycode='CHN' GROUP BY district;

# 输出一下格式
guangdong   guangzhou,shenzhen,foshan.... 
小扩展
anhui : hefei,huaian ....
SELECT CONCAT(district,":" ,GROUP_CONCAT(NAME)) FROM city WHERE countrycode='CHN' GROUP BY district ;

SELECT 配合 ORDER BY LIMIT 子句

统计所有国家的总人口数量, 将总人口数大于5000w的过滤出来,并且按照从大到小顺序排列

SELECT countrycode,SUM(population) FROM city GROUP BY countrycode HAVING SUM(population)>50000000 ORDER BY SUM(population) DESC;

统计所有国家的总人口数量,将总人口数大于5000w的过滤出来,并且按照从大到小顺序排列,只显示前三名

SELECT countrycode,SUM(population) FROM city
GROUP BY countrycode
HAVING SUM(population)>50000000
ORDER BY SUM(population) DESC 
LIMIT 3 OFFSET 0;LIMIT M,N     :跳过M行,显示一共N行

LIMIT Y OFFSET X: 跳过X行,显示一共Y行

SELECT countrycode,SUM(population) FROM city
GROUP BY countrycode
HAVING SUM(population)>50000000
ORDER BY SUM(population) DESC 
LIMIT 3 OFFSET 3

练习题

统计中国每个省的总人口数，只打印总人口数小于100w的

SELECT   district ,SUM(population)  FROM  city 
WHERE countrycode='CHN'
GROUP BY district
HAVING SUM(population)<1000000;

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 ,SUM(population)  FROM  city 
WHERE countrycode='CHN'
GROUP BY district
ORDER BY SUM(population) 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;

select disctrict , count(name) from city  
where countrycode='CHN'
group by  district 
having  count(name) >10
order by  count(name) desc 
limit 3;

union 和 union all

作用: 多个结果集合并查询的功能

区别

union all 不做去重复
union 会做去重操作

查询中或者美国的城市信息

SELECT * FROM city WHERE countrycode='CHN' OR countrycode='USA';
改写为:
SELECT * FROM city WHERE countrycode='CHN'
UNION ALL 
SELECT * FROM city WHERE countrycode='USA';

多表连接查询内连接

作用

单表数据不能满足查询需求时，连接多个表查询

数据准备

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;

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