mysql表间的关系和查询

mysql间的关系及查询

多对一

# 先创建被关联的表dep                
create table dep(id primary key auto_increment,
                 dep_name char(10),
                 dep_comment char(60))
                 
# 后创建关联表emp              
create table emp(id int primary key auto_increment,
                 name char(15),
                 gender enum('male','femael')not null default 'male',
                 de_id int,
                 foreign key(dep_id),references dep(id));
                 
# 先向被关联的表dep中插入值
insert into dep(dep_name,dep_comment) values('教学部','给学生上课',
                                             '外交部','代表学生的良好形象',
                                             '技术部','维护教学设备'));
                                             
# 再向关联表emp中插入值
insert into emp(name,gender,de_id) values(('aaa','male',1),
                                          ('bbb','male',2),
                                          ('ccc','female',3),
                                          ('ddd','male',2)
                                            );

多对多关系

表一：book

id	book_name	price
1	九阴真经	100
2	葵花宝典	200
3	吸星大法	300

表二：press

id	press_name	phone
1	人民出版社	123
2	青春文学	234
3	北京邮电出版社	345

一本书可以是多个出版社出版的,所以需要单独建一个关系表出来

id	book_id	p_id
1	1	1
2	1	3
3	2	1

创建表

# 创建表1
create table book(id int primary key auto_increment,
                  book_name char(15),
                  price int(5));
# 创建表2
create table press(id int primary key auto_increment,
                   press_name char(20),
                   phone int(11));                 
# 创建关系表
create table b_p(id int primary key auto_increment,
                 book_id int(3),
                 p_id int(3),
                foreign key(book_id) references book(id) on update cascade on delete cascade,
                foreign key(p_id) references press(id) on update cascade on delete cascade);

# 插入值
insert into book values(1,'九阴真经',100),(2,'九阳神功',200),(3,'吸星大法',300);

insert into press(press_name) values('人民出版社'),('青春文学'),('北京邮电出版社');

insert into b_p(book_id,p_id) values(1,1),(1,3),(2,1);

修改表和复制表

# 修改表名
    alter table 表名 rename 新表名;
    
    
# 增加新字段
    alter table 表名 add 字段名 数据类型[约束条件];
    

# 删除字段
    alter table 表名 drop 字段名
    
    
# 修改字段
    alter table 表名 modify 字段名 数据类型[约束条件];
    
    alter table 表名 change 旧字段名 新字段名 旧数据类型[约束条件];
    
    alter table 表名 change 旧字段名 新字段名 新数据类型[约束条件];
    
    
# 复制表
    create table book_copy select *from book;
    

单表查询

select id,name from emp
    where id >1 and name like'%on%'
    group by dep_id
    having 分组后的过滤条件
    order by 排序依据
    limit n;

group by & 聚合函数

# 设置sql_mode 为 only_full_group_by

select 字段1 from 表名 group by 字段1     # 注意前后两个字段名要相同

# 每个部门员工的最高工资
select post,max(salary) from emp group by post;

select post,min(salary) from emp group by post;

select post,avg(salary) from emp group by post;

select post,sum(salary) from emp group by post;

select post,count(gender) from emp group by post;

select post,avg(salary) from emp where age>=30 group by post;

group_concat & 拼接字符串

select post,group_concat(name) from emp group by post;

select post,group_concat(name,'sb')from emp group by post;

select post,group_concat(name,":",salary)from emp group by post;

concat & as

select concate('姓名',name) from emp ;

select name,salary*12 as annual_salary from emp;

select concate('姓名',name) as 姓名  from emp;

having

# 1. having 的语法给是与where一模一样，但是having是在group by之后进行一步过滤
# 2. where不能用聚合函数，但是having可以用聚合函数

# 统计个部门年龄在30岁以上的员工的平均工资，并且保留平均工资大于10000的部门
select post,avg(salary) from emp
                        where age>=30
                        group by post
                        having avg(salary)>10000;
            
# 强调： having必须在group by后用

distinct

# 去重，执行顺序在having之后
select distinct post,avg(salary) from emp
        where age>=30
        group by post
        having avg(salary)>10000;

order by

select * from emp order by salary;      # 默认升序（asc)排

select * from emp order by salary desc; # 降序排

select * from emp order by age desc,salary asc; # 多个排序标准，

# 统计个部门年龄大于10岁，平均工资大于1000，且升序排列
select distinct post,avg(salary)from emp where age >10 
            group by post
            having avg(salary) > 1000
            order by post;

limit

# 限制显示条数

select * from emp limit 3;

# 选择工资前三的员工信息
select * from emp order by salary desc limit 3;

# 分页显示
select * from emp limit 5,5;

正则匹配

select * from emp where name regexp '^jin.*(n|g)$';

# $表示结束
# regexp关键字

多表连接

建表

create table department(id int,name varchar(20));

create table employee(id int primary key auto_increment,
                      name varchar(20),sex enum('male','female')not null default 'male',age intm dep_id int);

笛卡尔积查询

select * from emp,dep where emp.dep_id = dep.id

内连接

select * from emp inner join dep on emp.dep_id = dep.id;

左连接

select * from emp left join dep on emp.dep_id = dep.id;

右连接

select * from right join dep on emp.dep_id = dep.id;

全连接

select * from emp left join dep on emp.dep_id = dep.id
union
select * from right join dep on emp.dep_id = dep.id;

子查询

select * from emp inner join dep on demp.dep_id = dep.id
select name from emp where dep_id = (select id from dep where name = '技术');