第十九周-day83-数据库day05

1. join多表查询

1.1 作用

业务需要带的数据来自多张表时

1.2 多表连接类型

内连接 ☆☆☆☆☆
外连接 ☆☆☆
全连接
笛卡尔

1.3 内连接的类型

传统连接 where
自连接
join uing	☆☆
join on		☆☆☆☆☆

1.4 join on 的语法

表A和表B横向拼成 一行

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'

#语法
select xxx 
from A
join B
on A.xxx= B.yyy
where
group by
having
order
limit

1. 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

多表连接的套路：
1.根据需求找到关联表
2.找到表与表的关联列

2. 多表连接案例

#进入world库练习
use world;

2.1查询一下世界上人口数量小于100人的城市名，国家名，国土面积和人口数

SELECT country.name,country.SurfaceArea,city.name,city.Population
FROM city
JOIN country
ON city.CountryCode=country.code
WHERE city.Population<100;

2.2 打开school表

2.2.1 查询zhangs学习了几门课程

SELECT student.sname,COUNT(sc.cno)
FROM student
JOIN sc
ON student.sno=sc.sno
WHERE student.sname='zhang3';

2.2.2 统计zhang3学习课程名称

SELECT student.sname,GROUP_CONCAT(course.cname)
FROM student
JOIN sc
ON student.sno=sc.sno
JOIN course
ON sc.cno=course.cno
WHERE student.sname='zhang3'
GROUP BY student.sno;

2.2.3 oldguo老师教了学生的个数

SELECT teacher.tname,GROUP_CONCAT(student.sname),COUNT(student.sname)
FROM student
JOIN sc
ON student.sno=sc.sno
JOIN course
ON sc.cno=course.cno
JOIN teacher
ON course.tno=teacher.tno
WHERE teacher.tname='oldguo'
GROUP BY teacher.tno;

2.2.4 每位老师所教课程的平均分,并按平均分排序

SELECT teacher.tname,AVG(sc.score)
FROM teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
GROUP BY teacher.tno
ORDER BY AVG(sc.score) DESC;

2.2.5 查询oldguo所教的不及格的学生姓名

SELECT teacher.tname,student.sname,sc.score
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' AND sc.score<60;

2.2.6 查询所有老师所教学生不及格的信息

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 sc.score<60
GROUP BY teacher.tname

3.别名的使用

SELECT 别名.列
FROM 表 表别名
WHERE 别名.列
GROUP BY 别名.列
HAVING 列别名
ORDER BY 列别名

3.1 表别名

SELECT a.tname,GROUP_CONCAT(d.sname)
FROM teacher AS a
JOIN course AS b
ON a.tno=b.tno
JOIN sc AS c
ON b.cno=c.cno
JOIN student AS d 
ON c.sno=d.sno
WHERE c.score<60
GROUP BY a.tname

说明：表别名一般是在FROM的表的表名，或者join后的表的别名
在 WHERE,GROUP BY,select后的列，having,ORDER BY

3.2 列别名

SELECT a.tname AS 讲师 ,GROUP_CONCAT(d.sname) AS 学生
FROM teacher AS a
JOIN course AS b
ON a.tno=b.tno
JOIN sc AS c
ON b.cno=c.cno
JOIN student AS d 
ON c.sno=d.sno
WHERE c.score<60
GROUP BY a.tname

说明：列别名一般是在select后的列，定义的别名
– 作用：
– 1.结果集显示会以别名形式展示
– 2.在having 和order by 中可以调用列别名

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

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4.外链接简介☆☆☆

left join 左链接
right join 右链接

A left join B
on A.x=B.y
where

A left join B
on A.x=B.y
and

结论：

1. 多表连接中，小标驱动大表
2. 通过left join 强制选定驱动表

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工作中还需要学习的内容：
1.内置函数
2.存储过程
3.函数
4.触发器
5.事件
6.视图
7.JSON语法
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5.元数据获取

5.1 元数据介绍

”基表“ ：数据字典信息(列结构 frm)，系统状态，对象状态

相当于linux中的 inode

DDL DCL语句修改元数据

5.1 show语句(MySQL独家)

help show;
show  databases;                        #查看所有数据库
show tables;                            #查看当前库的所有表
SHOW TABLES FROM                        #查看某个指定库下的表
show create database world              #查看建库语句
show create table world.city            #查看建表语句
show  grants for  root@'localhost'      #查看用户的权限信息
show  charset；                         #查看字符集
show collation                          #查看校对规则
show processlist;                       #查看数据库连接情况
show index from                         #表的索引情况
show status                             #数据库状态查看
SHOW STATUS LIKE '%lock%';         		#模糊查询数据库某些状态
SHOW VARIABLES                          #查看所有配置信息
SHOW variables LIKE '%lock%';          	#查看部分配置信息
show engines                            #查看支持的所有的存储引擎
show engine innodb status\G             #查看InnoDB引擎相关的状态信息
show binary logs                        #列举所有的二进制日志
show master status                      #查看数据库的日志位置信息
show binlog evnets in                   #查看二进制日志事件
show slave status \G                    #查看从库状态
SHOW RELAYLOG EVENTS               		#查看从库relaylog事件信息
desc  (show colums from city)           #查看表的列定义信息

5.2 information_schema.tables 虚拟库

information_schema —> VIEWS 视图

use information_schema;
show tables;

CREATE VIEW test AS SELECT    
country.name AS co_name,country.SurfaceArea,city.name AS ci_name,city.Population
FROM city   JOIN country
ON city.CountryCode=country.code
WHERE city.Population<100;

SELECT * FROM test;

5.2.1 TABLES 作用和结构

作用: 存储整个数据库中,所有表的元数据的查询方式.

desc tables;	
TABLE_SCHEMA    表所在的库   
TABLE_NAME     	表名
ENGINE          表的引擎
TABLE_ROWS     	表的行数
AVG_ROW_LENGTH 	平均行长度
INDEX_LENGTH   	索引长度

例子

1.查询world 数据库下的所有表名
use world;
show tables;	&& show tables from world;

2.查询整个数据库下的所有表名
select table_name from information_schema.tables;

3.查询所有InnoDB引擎的表
SELECT table_schema,table_name,ENGINE FROM information_schema.tables
WHERE ENGINE='innodb';

4.统计每张表的实际的空间占用大小情况
(AVG_ROW_LENGTH*TABLE_ROWS+INDEX_LENGTH)

SELECT table_name,AVG_ROW_LENGTH*TABLE_ROWS+INDEX_LENGTH
FROM information_schema.tables;

5.统计每个库的空间使用大小情况
SELECT table_schema,SUM(AVG_ROW_LENGTH*TABLE_ROWS+INDEX_LENGTH)/1024/1024
FROM information_schema.tables
GROUP BY table_schema;

中小型公司：数据量大小一般在 300G 500G

中大型公司：数据量大小一般在 2T 10T

6.对MySQL的数据库进行分库分表备份
mysqldump -uroot -p123 world city >/backup/world_city.sql

SELECT CONCAT("mysqldump -uroot -p123 ",table_schema ," ",table_name ," >/backup/",table_schema,
"_",table_name,".sql")
FROM information_schema.tables INTO OUTFILE '/tmp/bak.sql';

添加配置

7.模仿模板语句，批量生成对world数据库下的表操作语句
alter table world.city discard tablespace;

SELECT CONCAT("alter table ",table_schema,".",table_name," discard tablespace;")
FROM information_schema.tables
WHERE table_schema='world'
INTO OUTFILE '/tmp/discard.sql';

8. 模仿模板语句,批量生成对world数据库下的表操作的语句
atler table world.city engine=innodb;

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