# 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
```sql
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
```

![[Pasted image 20230110121247.png]]

![[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
```sql
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
```sql
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;
```