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# Database Management

## 1. Database Management

### 1.1 Create a Database

This command is used to create a database.

**Syntax:**

```SQL
 CREATE DATABASE (IF NOT EXISTS)? <DATABASE_NAME> (WITH properties)?
```

**Note: **

1. `<DATABASE_NAME>`: The name of the database, with the following characteristics:
  - Case-insensitive. After creation, it will be displayed uniformly in lowercase.
  - Can include commas (`,`), underscores (`_`), numbers, letters, and Chinese characters.
  - Maximum length is 64 characters.
  - Names with special characters or Chinese characters must be enclosed in double quotes (`""`).

2. `WITH properties`: Property names are case-insensitive. For more details, refer to the case sensitivity rules [case-sensitivity](../SQL-Manual/Identifier.md#2-case-sensitivity)。Configurable properties include:

| Property                | Description                                                  | Default Value        |
| ----------------------- | ------------------------------------------------------------ | -------------------- |
| TTL                     | Automatic data expiration time, in milliseconds              | `INF`                |
| TIME_PARTITION_INTERVAL | Time partition interval for the database, in milliseconds    | `604800000` (7 days) |
| SCHEMA_REGION_GROUP_NUM | Number of metadata replica groups; generally does not require modification | `1`                  |
| DATA_REGION_GROUP_NUM   | Number of data replica groups; generally does not require modification | `2`                  |

**Examples:**

```SQL
CREATE DATABASE IF NOT EXISTS database1 with(TTL=31536000000);
```

### 1.2 Use a Database

Specify the current database as the namespace for table operations.

**Syntax:**

```SQL
USE <DATABASE_NAME>
```

**Example:** 

```SQL
USE database1;
```

### 1.3 View the Current Database

Displays the name of the currently connected database. If no USE statement has been executed, the default is `null`.

**Syntax:**

```SQL
SHOW CURRENT_DATABASE
```

**Example:**

```SQL
USE database1;
SHOW CURRENT_DATABASE;
```
```shell
+---------------+
|CurrentDatabase|
+---------------+
|      database1|
+---------------+
```


### 1.4 View All Databases

Displays all databases and their properties.

**Syntax:**

```SQL
SHOW DATABASES (DETAILS)?
```

**Columns Explained:**


| Column Name             | Description                                                                                                                                                                                                                                                                                                                                   |
| ----------------------- |-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| database                | Name of the database.                                                                                                                                                                                                                                                                                                                         |
| TTL                     | Data retention period. If TTL is specified when creating a database, it applies to all tables within the database. You can also set or update the TTL of individual tables using [create table](../Basic-Concept/Table-Management_apache.md#11-create-a-table) 、[alter table](../Basic-Concept/Table-Management_apache.md#14-update-tables) . |
| SchemaReplicationFactor | Number of metadata replicas, ensuring metadata high availability. This can be configured in the `iotdb-system.properties` file under the `schema_replication_factor` property.                                                                                                                                                                |
| DataReplicationFactor   | Number of data replicas, ensuring data high availability. This can be configured in the `iotdb-system.properties` file under the `data_replication_factor` property.                                                                                                                                                                          |
| TimePartitionInterval   | Time partition interval, determining how often data is grouped into directories on disk. The default is typically one week.                                                                                                                                                                                                                   |
| Model                   | Returned when using the `DETAILS` option, showing the data model corresponding to each database (e.g., timeseries tree model or device table model).                                                                                                                                                                                          |

**Examples:** 

```SQL
SHOW DATABASES DETAILS;
```
```shell
+------------------+-------+-----------------------+---------------------+---------------------+--------------------+------------------+
|          Database|TTL(ms)|SchemaReplicationFactor|DataReplicationFactor|TimePartitionInterval|SchemaRegionGroupNum|DataRegionGroupNum|
+------------------+-------+-----------------------+---------------------+---------------------+--------------------+------------------+
|         database1|    INF|                      1|                    1|            604800000|                   1|                 2|
|information_schema|    INF|                   null|                 null|                 null|                null|              null|
+------------------+-------+-----------------------+---------------------+---------------------+--------------------+------------------+
```

### 1.5 Update a Database

Used to modify some attributes in the database.

**Syntax:**

```SQL
ALTER DATABASE (IF EXISTS)? database=identifier SET PROPERTIES propertyAssignments
```

**Note:**

1. The `ALTER DATABASE` operation currently only supports modifications to the database's `SCHEMA_REGION_GROUP_NUM`, `DATA_REGION_GROUP_NUM`, and `TTL` attributes.

**Example:**

```SQL
ALTER DATABASE database1 SET PROPERTIES TTL=31536000000;
```

### 1.6 Delete a Database

Deletes the specified database and all associated tables and data.

**Syntax:**

```SQL
DROP DATABASE (IF EXISTS)? <DATABASE_NAME>
```

**Note:**

1. A database currently in use can still be dropped.
2. Deleting a database removes all its tables and stored data.

**Example:**

```SQL
DROP DATABASE IF EXISTS database1;
```