6.Mysql之基本SQL语法(03)

1.前言：

　　这小结主要是用来细说多表连接(多表查询)，其中多表查询是DBA开发与运维中是重要的一环，对于从事DBA的人员来说，该技能是必须要掌握的，然后是该小结也是Mysql之基本SQL语法的最后小结

2. 多表查询

　　2.1语法格式

select 表名.列 ...from  表名1  
join 表名2 on 表名1.列1=表名2.列2
where ...
group by  ...
having  ...
order by  ...
Limit  ...

　　2.2 一定要按照以上顺序语法进行书写，格式首先要正确

　　2.3多张表进行关联语法：A join b on A.x=b.y join C on b.m=C.n

3.示例

　　3.1 ER图如下：

　　　　

　　3.2 整体实例如下：　

1.创建一个数据库school
create database school charset utf8mb4;

2.创建学生表(student)
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;

3.创建教师表(teacher)
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;

4.创建课程表(course)
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;

5.创建成绩表(score)
CREATE TABLE score (
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;

6.插入数据：
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);

==============================

INSERT INTO score(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);

　　练习：

1.统计张三学了几门课程
select sname, count(cno) from student join sc on student.sno=sc.sno where sname='zhang3';

select student.sname, count(sc.cno) from student join sc on student.sno=sc.sno where student.sname='zhang3';

2.查询oldguo老师教的学生名.
select 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';


3.查询zhang3 学习的课程名称有哪些？
select student.sname,course.cname from
student join sc on student.sno=sc.sno
join course on sc.cno=course.cno
where student.sname='zhang3';

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.sname;

4.查询oldguo所教课程的平均分数
select course.cname,avg(sc.score) from
teacher join course on teacher.tno=course.tno join
sc on course.cno=sc.cno
where teacher.tname='oldboy';


5.每位老师所教课程的平均分，并按平均分排序
select teacher.tname,course.cname,avg(sc.score) from
teacher join course on teacher.tno=course.tno join
sc on course.cno=sc.cno
group by teacher.tname order by avg(sc.score);

6.查询oldguo所教的不及格的学生姓名
select 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 sc.score<60 and teacher.tname='oldguo';


7.查询所有老师所教学生不及格的信息
select teacher.tname,student.sname,course.cname,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 sc.score<60 group by student.sname;

4.元数据信息查询

　　4.0 Mysql元数据信息包括很多内容：表的大小、表的创建时间、那个用户创建的该表、该表中的数据页的信息以及索引页的一些信息等等，今天这里主要将的是关于information_schema

下面的tables表，该表信息量很大且很有用，以下就是根据该表进行统计的一些查询信息。

root@localhost 00:37:  [information_schema]> select * from tables limit 1 G;
*************************** 1. row ***************************
  TABLE_CATALOG: def
   TABLE_SCHEMA: information_schema
     TABLE_NAME: CHARACTER_SETS
     TABLE_TYPE: SYSTEM VIEW
         ENGINE: MEMORY
        VERSION: 10
     ROW_FORMAT: Fixed
     TABLE_ROWS: NULL
 AVG_ROW_LENGTH: 384
    DATA_LENGTH: 0
MAX_DATA_LENGTH: 16434816
   INDEX_LENGTH: 0
      DATA_FREE: 0
 AUTO_INCREMENT: NULL
    CREATE_TIME: 2021-06-16 00:37:40
    UPDATE_TIME: NULL
     CHECK_TIME: NULL
TABLE_COLLATION: utf8_general_ci
       CHECKSUM: NULL
 CREATE_OPTIONS: max_rows=43690
  TABLE_COMMENT: 
1 row in set (0.03 sec)

　　4.1显示所有的库和表的信息

SELECT TABLE_SCHEMA ,TABLE_NAME from information_schema.tables;

SELECT table_schema,GROUP_CONCAT(TABLE_name) from information_schema.tables group BY TABLE_schema;

　　4.2查询一下innodb引擎的表

SELECT TABLE_SCHEMA ,TABLE_NAME from information_schema.tables where engine='innodb';

　　4.3.统计某个库下的某个表占用空间大小

公式：某个表占用空间大小=平均行长度*行数+索引长度

举例1：统计shcool库下的student表占用空间的大下
SELECT TABLE_name,(AVG_ROW_LENGTH*TABLE_ROWS+INDEX_LENGTH)/1024 from information_schema.tables WHERE TABLE_schema='shcool' and TABLE_name='student';

举例2: 统计shcool库下所有表的总大小(求这个库所占用的空间大下)
SELECT table_schema,SUM((AVG_ROW_LENGTH*TABLE_ROWS+INDEX_LENGTH)/1024) from information_schema.tables where TABLE_schema='shcool';

　　4.4 统计每个库的数据量大下，并按照数据量大下排序

SELECT  TABLE_schema,
SUM((AVG_ROW_LENGTH*TABLE_ROWS+INDEX_LENGTH)/1024)
as total_KB from information_schema.tables 
GROUP BY TABLE_schema order by total_KB DESC;

　　4.5.配合concat()函数拼接语句或命令 *****

　　　　4.5.1 模仿以下语句，进行数据库的分库分表备份　　　　

mysqldump -uroot -p123 world city >/bak/world_city.sql

SELECT concat( "mysqldump -uroot -p123"," ",table_schema," ",TABLE_name," >/bak/",TABLE_schema,"_",table_name,".sql")
from information_schema.tables

　　　　4.5.2模仿以下语句，进行批量生成对world库下所有表进行操作

ALTER TABLE world.city DISCARD TABLESPACE;

select concat("ALTER TABLE", TABLE_schema,".",table_name," DISCARD TABLESPACE") from information_schema.tables
WHERE TABLE_schema='shcool';

5.Show 命令总结

show databases;
show tables;
show create database xx
show create table xx :查看所有建表语句
show processlist :查看所有用户连接用户情况
show charset:
show collation:
show grants for xx： ：查看用户权限信息
show variables like '%xx%'; : 查看参数信息
show engine : 查看所有支持的存储引擎类型
show index from xxx : 查看表的索引信息
show engine innodb statusG :查看innodb引擎详细状态信息
show binary log :查看二进制日志的列表信息
show binlog events in '' :查看二进制日志的事件信息
show master status ; :查看mysql当前使用二进制日志信息
show slave statusG; :查看从库状态信息
show relaylog events in '' :查看中继日志的事件信息
show status like '' :查看数据整体状态信息