日常数据库操作

#### 一、数据库用量查询

##### **1.查看所有数据库各容量大小**

```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;
```

##### **2.查看所有数据库各表容量大小**

```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;
```

##### **3.查看指定数据库容量大小**

例：查看mysql库容量大小

```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

where table_schema='mysql';　
```

##### **4.查看指定数据库各表容量大小**

例：查看mysql库各表容量大小

```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

where table_schema='mysql'

order by data_length desc, index_length desc;
```

#### 二、数据库清理

##### 1.drop

drop 是直接删除表信息，速度最快，但是无法找回数据

```sql
drop table user;
```

##### 2.truncate

truncate 是删除表数据，不删除表的结构，速度排第二，但不能与where一起使用

```sql
truncate table user;
```

##### 3.delete from

delete 是删除表中的数据，不删除表结构，速度最慢，但可以与where连用，可以删除指定的行

删除user表的所有数据

```sql
delete from user;
```

删除user表的指定记录

```sql
delete from user where user_id = 1;
```

##### 4.用法总结

- 希望删除表结构时，用 drop;
- 希望保留表结构，但要删除所有记录时， 用 truncate;
- 希望保留表结构，但要删除部分记录时， 用 delete。