obsidian_note/技术探究/mysql.md

mysql 导出

mysqldump https://blog.csdn.net/ichen820/article/details/124022855

连接

连接mysql3306端口命令

mysql -h58.64.217.120 -ushop -p123456

连接非3306端口(指定其他端口) 的命令

mysql -h58.64.217.120 -P3308-ushop -p123456

分组

分组合并查询字段

SELECT
	GROUP_CONCAT(a.id) as ids,
	GROUP_CONCAT(a.name) as names,
	CASE
		WHEN a.age<18 THEN '少年'
		WHEN a.age>=18 AND a.age< 40 THEN '青年'
		WHEN a.age>=40 AND a.age< 60 THEN '中年'
	END as age,
	count(a.id) as count
FROM
	(SELECT id, name, birthday, TIMESTAMPDIFF(YEAR,student.birthday,CURDATE()) as age FROM `student`) as a
GROUP BY
	CASE
		WHEN a.age<18 THEN '少年'
		WHEN a.age>=18 AND a.age< 40 THEN '青年'
		WHEN a.age>=40 AND a.age< 60 THEN '中年'
	END

!

!SQL原理.html

修改表字符集

alter  table t2 convert to character set utf8mb4;

查看所有表大小

select  
table_schema as '数据库',  
table_name as '表名',  
table_rows as '记录数',  
truncate(data_length/1024/1024, 2) as '数据容量(MB)',  
truncate(index_length/1024/1024, 2) as '索引容量(MB)'  
from information_schema.tables  
order by data_length desc, index_length desc;

查看所有库大小

select  
table_schema as '数据库',  
sum(table_rows) as '记录数',  
sum(truncate(data_length/1024/1024, 2)) as '数据容量(MB)',  
sum(truncate(index_length/1024/1024, 2)) as '索引容量(MB)'  
from information_schema.tables  
group by table_schema  
order by sum(data_length) desc, sum(index_length) desc;