Mysql8.0安装以及递归表达式使用

mysql安装

以centos7为例安装！

安装wget

yum install wget -y

给CentOS添加rpm源，并且选择较新的源

官网地址

wget dev.mysql.com/get/mysql84-community-release-el7-1.noarch.rpm

下载安装的rpm源文件

yum install mysql84-community-release-el7-1.noarch.rpm -y

使用yum安装mysql

yum install mysql-community-server -y

mysql相关的设置

启动服务以及开机自启

#启动mysql服务
service mysqld start
#设置mysql开机启动
chkconfig mysqld on

修改密码

##查看临时密码
grep "password" /var/log/mysqld.log
## 临时密码登录
mysql -uroot -p
##修改密码策略和密码
set global validate_password.policy=LOW;
set global validate_password.length=1;
set global validate_password.special_char_count=0;
set global validate_password.mixed_case_count=0 ;
set global validate_password.number_count =0;ALTER USER 'root'@'localhost' IDENTIFIED BY 'rootA@123456';

授权 8.0有区别

#创建用户
create user 'echo'@'%' identified by '123456';
# 授权
grant all privileges on *.* to 'echo'@'%' with grant option;
# 刷新权限
FLUSH PRIVILEGES;

递归表达式ETC

递归表达式是一种特殊的子查询，它能够引用其本身的查询结果。它包含两部分

SELECT ...      -- return initial row set
UNION ALL
SELECT ...      -- return additional row sets

例如：

WITH RECURSIVE cte (n) AS
(SELECT 1UNION ALLSELECT n + 1 FROM cte WHERE n < 5
)
SELECT * FROM cte;

典型应用

斐波那契数列

WITH RECURSIVE fibonacci (n, fib_n, next_fib_n) AS
(SELECT 1, 0, 1UNION ALLSELECT n + 1, next_fib_n, fib_n + next_fib_nFROM fibonacci WHERE n < 10
)
SELECT * FROM fibonacci;

时间序列生成

WITH RECURSIVE dates (date) AS
(SELECT MIN(date) FROM salesUNION ALLSELECT date + INTERVAL 1 DAY FROM datesWHERE date + INTERVAL 1 DAY <= (SELECT MAX(date) FROM sales)
)
SELECT * FROM dates;

层次结构遍历

以评论回复为例

WITH RECURSIVE CommentHierarchy AS (SELECT c.comment_id,c.post_id,c.user_id,c.parent_id,c.content,c.created_at,c.updated_at,0 AS levelsFROM Comments cWHERE c.parent_id IS NULLUNION ALLSELECT c.comment_id,c.post_id,c.user_id,c.parent_id,CONCAT(ch.content,'->',c.content) content,c.created_at,c.updated_at,ch.levels + 1 AS levelsFROM Comments cINNER JOIN CommentHierarchy ch ON c.parent_id = ch.comment_id
)
SELECT * FROM CommentHierarchy ORDER BY post_id, levels, created_at;

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