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---
toc_max_heading_level: 4
title: "View"
sidebar_label: "View"
---
## Introduction
Starting from TDengine 3.2.1.0, TDengine Enterprise Edition provides view function, which is convenient for users to simplify operation and improve sharing ability among users.
A view is essentially a query statement stored in a database. The view (non-materialized view) itself does not contain data, and only dynamically executes the query statement specified by the view when reading data from the view. We specify a name when creating a view, and then we can query it like using a regular table. The use of views should follow the following rules:
- Views can be defined and used nested, and are bound to the specified or current database when created.
- Within the same database, duplicate view names are not allowed, and it is recommended not to have duplicate view names and table names (not mandatory). When the view and table names have the same name, operations such as writing, querying, authorizing, and revoking permissions will prioritize using the same-named table.
## Grammar
### Create (update) view
```sql
CREATE [ OR REPLACE ] VIEW [db_name.]view_name AS query
```
Description:
- When creating a view, you can specify the database name ( db_name ) to which the view is bound. If not explicitly specified, it defaults to the database bound to the current connection.
- It is recommended to specify the database name in the query statement, support cross-database views, and default to the database bound to the view when not specified (it may not be the database specified by the current connection);
### View View
1. View all views under a database
```sql
SHOW [db_name.]VIEWS;
```
2. View the creation statement of the view
```sql
SHOW CREATE VIEW [db_name.]view_name;
```
3. View view column information
```sql
DESCRIBE [db_name.]view_name;
```
4. View all view information
```sql
SELECT ... FROM information_schema.ins_views;
```
### Delete a view
```sql
DROP VIEW [IF EXISTS] [db_name.]view_name;
```
## Permissions
### Description
View permissions are divided into three types: READ, WRITE, and ALTER. Query operations require READ permissions, write operations require WRITE permissions, and delete and modify operations on the view itself require ALTER permissions.
### Rules
- The creator of the view and the root user have all permissions by default.
- Authorization and revocation of permissions for other users can be performed through the GRANT and REVOKE statements, which can only be performed by the root user.
- View permissions need to be authorized and revoked separately. Authorization and revocation through db. * do not include view permissions.
- Views can be defined and used nested, and the verification of view permissions is also performed by recursion.
- In order to facilitate the sharing and use of views, the concept of view effective user (i.e. the user who creates the view) is introduced. Authorized users can use the read and write permissions of the view effective user's library, table, and nested view. Note: After the view is REPLACE, the effective user will also be updated.
The detailed rules for controlling relevant permissions are summarized as follows:
| Serial number | Operation | Permission requirements |
| --- | --- | --- |
| 1 | CREATE OR REPLACE VIEW (Create a new view) | The user has WRITE permission on the database to which the view belongs And Users have query permissions for the target library, table, and view of the view. If the object in the query is a view, it must meet rule 8 in the current table. |
| 2 | CREATE OR REPLACE VIEW (Overwrite old view) | The user has WRITE permission on the database to which the view belongs, and ALTER permission on the old view And Users have query permissions for the target library, table, and view of the view. If the object in the query is a view, it must meet rule 8 in the current table. |
| 3 | DROP VIEW | The user has ALTER permission on the view |
| 4 | SHOW VIEWS | No |
| 5 | SHOW CREATE VIEW | No |
| 6 | DESCRIBE VIEW | No |
| 7 | System table query | No |
| 8 | SELECT FROM VIEW | The operating user has READ permissions for the view And Operating users or view effective users have READ permissions on the target library, table, and view of the view |
| 9 | INSERT INTO VIEW | The operation user has WRITE permission on the view And Operating users or view effective users have WRITE permissions on the target library, table, and view of the view |
| 10 | GRANT/REVOKE | Only the root user has permission |
### Grammar
#### Authorization
```sql
GRANT privileges ON [db_name.]view_name TO user_name
privileges: {
ALL,
| priv_type [, priv_type] ...
}
priv_type: {
READ
| WRITE
| ALTER
}
```
#### Recover permissions
```sql
REVOKE privileges ON [db_name.]view_name FROM user_name
privileges: {
ALL,
| priv_type [, priv_type] ...
}
priv_type: {
READ
| WRITE
| ALTER
}
```
## Usage scenarios
| SQL query | SQL write | STMT query | STMT write | Subscribe | Stream |
| --- | --- | --- | --- | --- | --- |
| Support | Not supported yet | Not supported yet | Not supported yet | Support | Not supported yet |
## Example
- Create a view
```sql
CREATE VIEW view1 AS SELECT _wstart, count(*) FROM table1 INTERVAL(1d);
CREATE VIEW view2 AS SELECT ts, col2 FROM table1;
CREATE VIEW view3 AS SELECT * from view1;
```
- Query data
```sql
SELECT * from view1;
```
- Delete data
```sql
DROP VIEW view1;
```

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---
toc_max_heading_level: 4
title: "视图"
sidebar_label: "视图"
---
## 简介
从 TDengine 3.2.1.0 开始，TDengine 企业版提供视图功能，便于用户简化操作，提升用户间的分享能力。
视图（View）本质上是一个存储在数据库中的查询语句。视图（非物化视图）本身不包含数据，只有在从视图读取数据时才动态执行视图所指定的查询语句。我们在创建视图时指定一个名称，然后可以像使用普通表一样对其进行查询等操作。视图的使用需遵循以下规则：
- 视图可以嵌套定义和使用，视图与创建时指定的或当前数据库绑定使用。
- 在同一个数据库内，视图名称不允许重名，视图名跟表名也推荐不重名（不强制）。当出现视图与表名重名时，写入、查询、授权、回收权限等操作优先使用同名表。
## 语法
### 创建（更新）视图
```sql
CREATE [ OR REPLACE ] VIEW [db_name.]view_name AS query
```
说明：
- 创建视图时可以指定视图绑定的数据库名（db_name），未明确指定时默认为当前连接绑定的数据库；
- 查询语句（query）中推荐指定数据库名，支持跨库视图，未指定时默认为与视图绑定的数据库(有可能非当前连接指定的数据库)；
### 查看视图
1. 查看某个数据库下的所有视图
```sql
SHOW [db_name.]VIEWS;
```
2. 查看视图的创建语句
```sql
SHOW CREATE VIEW [db_name.]view_name;
```
3. 查看视图列信息
```sql
DESCRIBE [db_name.]view_name;
```
4. 查看所有视图信息
```sql
SELECT ... FROM information_schema.ins_views;
```
### 删除视图
```sql
DROP VIEW [IF EXISTS] [db_name.]view_name;
```
## 权限
### 说明
视图的权限分为 READ、WRITE、ALTER 三种，查询操作需要具备 READ 权限，写入操作需要具备 WRITE 权限，对视图本身的删改操作需要具备 ALTER 权限。
### 规则
- 视图的创建者和 root 用户默认具备所有权限。
- 对其他用户进行授权与回收权限可以通过 GRANT 和 REVOKE 语句进行，该操作只能由 root 用户进行。
- 视图权限需单独授权与回收，通过db.*进行的授权与回收不含视图权限。
- 视图可以嵌套定义与使用，同理对视图权限的校验也是递归进行的。
- 为了方便视图的分享与使用，引入视图有效用户（即视图的创建用户）的概念，被授权用户可以使用视图有效用户的库、表及嵌套视图的读写权限。注：视图被 REPLACE 后有效用户也会被更新。
具体相关权限控制细则如下表所示：
| 序号 | 操作 | 权限要求 |
| ---- | --------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------- |
| 1 | CREATE VIEW <br/>(创建新用户) | 用户对视图所属数据库有 WRITE 权限<br/>且<br/> 用户对视图的目标库、表、视图有查询权限，若查询中的对象是视图需满足当前表中第8条规则 |
| 2 | CREATE OR REPLACE VIEW <br/>(覆盖旧视图) | 用户对视图所属数据库有 WRITE 权限 且 对旧有视图有 ALTER 权限 <br/>且<br/> 用户对视图的目标库、表、视图有查询权限，若查询中的对象是视图需满足当前表中第8条规则 |
| 3 | DROP VIEW | 用户对视图有 ALTER 权限 |
| 4 | SHOW VIEWS | 无 |
| 5 | SHOW CREATE VIEW | 无 |
| 6 | DESCRIBE VIEW | 无 |
| 7 | 系统表查询 | 无 |
| 8 | SELECT FROM VIEW | 操作用户对视图有 READ 权限 且 操作用户或视图有效用户对视图的目标库、表、视图有 READ 权限 |
| 9 | INSERT INTO VIEW | 操作用户对视图有 WRITE 权限 且 操作用户或视图有效用户对视图的目标库、表、视图有 WRITE 权限 |
| 10 | GRANT/REVOKE | 只有 root 用户有权限 |
### 语法
#### 授权
```sql
GRANT privileges ON [db_name.]view_name TO user_name
privileges: {
ALL,
| priv_type [, priv_type] ...
}
priv_type: {
READ
| WRITE
| ALTER
}
```
#### 回收权限
```sql
REVOKE privileges ON [db_name.]view_name FROM user_name
privileges: {
ALL,
| priv_type [, priv_type] ...
}
priv_type: {
READ
| WRITE
| ALTER
}
```
## 使用场景
| SQL 查询 | SQL 写入 | STMT 查询 | STMT 写入 | 订阅 | 流计算 |
| -------- | -------- | --------- | --------- | ---- | -------- |
| 支持 | 暂不支持 | 暂不支持 | 暂不支持 | 支持 | 暂不支持 |
## 举例
- 创建视图
```sql
CREATE VIEW view1 AS SELECT _wstart, count(*) FROM table1 INTERVAL(1d);
CREATE VIEW view2 AS SELECT ts, col2 FROM table1;
CREATE VIEW view3 AS SELECT * from view1;
```
- 查询数据
```sql
SELECT * from view1;
```
- 删除视图
```sql
DROP VIEW view1;
```