SQL is used by TDengine as the language for query. Application programs can send SQL statements to TDengine through REST API or connectors. TDengine CLI `taos` can also be used to execute SQL Ad-Hoc query. Here is the list of major query functionalities supported by TDengine：
For example, below SQL statement can be executed in TDengine CLI `taos` to select the rows whose voltage column is bigger than 215 and limit the output to only 2 rows.
```sql
select * from d1001 where voltage > 215 order by ts desc limit 2;
```
```title=Output
taos> select * from d1001 where voltage > 215 order by ts desc limit 2;
To meet the requirements in IoT use cases, some special functions have been added in TDengine, for example `twa` (Time Weighted Average), `spared` (The difference between the maximum and the minimum), `last_row` (the last row), more and more functions will be added to better perform in IoT use cases. Furthermore, continuous query is also supported in TDengine.
具体的查询语法请看 [TAOS SQL 的数据查询](/taos-sql/select) 章节。
For detailed query syntax please refer to [Select](/taos-sql/select).
In IoT use cases, there are always multiple data collecting points of same kind. A new concept, called STable (abbreviated for super table), is used in TDengine to represent a kind of data collecting points, and a table is used to represent a specific data collecting point. Tags are used by TDengine to represent the static properties of data collecting points. A specific data collecting point has its own values for static properties. By specifying filter conditions on tags, join query can be performed efficiently between all the tables belonging to same stable, i.e. same kind of data collecting points, can be. Aggregate functions applicable for tables can be used directly on stables, syntax is exactly same.
### 示例一
### Example 1
在 TAOS Shell，查找北京所有智能电表采集的电压平均值，并按照 location 分组。
In TDengine CLI `taos`, use below SQL to get the average voltage of all the meters in BeiJing grouped by location.
```
taos> SELECT AVG(voltage) FROM meters GROUP BY location;
...
...
@@ -61,9 +68,9 @@ taos> SELECT AVG(voltage) FROM meters GROUP BY location;
Join query is allowed between only the tables of same stable. In [Select](/taos-sql/select), all query operations are marked as whether it supports stable or not.
In IoT use cases, down sampling is widely used to aggregate the data by time range. `INTERVAL` keyword in TDengine can be used to simplify the query by time window. For example, below SQL statement can be used to get the sum of current every 10 seconds from meters table d1001.
```
taos> SELECT sum(current) FROM d1001 INTERVAL(10s);
...
...
@@ -88,7 +95,7 @@ taos> SELECT sum(current) FROM d1001 INTERVAL(10s);
Query OK, 2 row(s) in set (0.000883s)
```
降采样操作也适用于超级表，比如：将北京所有智能电表采集的电流值每秒钟求和
Down sampling can also be used for stable. For example, below SQL statement can be used to get the sum of current from all meters in BeiJing.
```
taos> SELECT SUM(current) FROM meters where location like "Beijing%" INTERVAL(1s);
...
...
@@ -102,7 +109,7 @@ taos> SELECT SUM(current) FROM meters where location like "Beijing%" INTERVAL(1s
Down sample also supports time offset. For example, below SQL statement can be used to get the sum of current from all meters but each time window must start at the boundary of 500 milliseconds.
```
taos> SELECT SUM(current) FROM meters INTERVAL(1s, 500a);
In IoT use cases, it's hard to align the timestamp of the data collected by each collecting point. However, a lot of algorithms like FFT require the data to be aligned with same time interval and application programs have to handle by themselves in many systems. In TDengine, it's easy to achieve the alignment using down sampling.
如果一个时间间隔里，没有采集的数据，TDengine 还提供插值计算的功能。
Interpolation can be performed in TDengine if there is no data in a time range.
For more details please refer to [Aggregate by Window](/taos-sql/interval).
## 示例代码
## Examples
### 查询数据
### Query
在 [SQL 写入](/develop/insert-data/sql-writing) 一章，我们创建了 power 数据库，并向 meters 表写入了一些数据，以下示例代码展示如何查询这个表的数据。
In [Insert](/develop/insert-data/sql-writing), a database named `power` is created and some data are inserted into stable `meters`. Below sample code demonstrates how to query the data in this stable.
<Tabs defaultValue="java" groupId="lang">
<TabItem label="Java" value="java">
...
...
@@ -154,16 +161,16 @@ Query OK, 5 row(s) in set (0.001521s)
1. With either REST connection or native connection, the above sample code work well.
2. Please be noted that `use db` can't be used in case of REST connection because it's stateless.
:::
### 异步查询
### Asynchronous Query
除同步查询 API 之外，TDengine 还提供性能更高的异步调用 API 处理数据插入、查询操作。在软硬件环境相同的情况下，异步 API 处理数据插入的速度比同步 API 快 2-4 倍。异步 API 采用非阻塞式的调用方式，在系统真正完成某个具体数据库操作前，立即返回。调用的线程可以去处理其他工作，从而可以提升整个应用的性能。异步 API 在网络延迟严重的情况下，优点尤为突出。
Besides synchronous query, asynchronous query API is also provided by TDengine to insert or query data more efficiently. With similar hardware and software environment, async API is 2~4 times faster than sync APIs. Async API works in non-blocking mode, which means an operation can be returned without finishing so that the calling thread can switch to other works to improve the performance of the whole application system. Async APIs perform especially better in case of poor network.
需要注意的是，只有使用原生连接的连接器，才能使用异步查询功能。
Please be noted that async query can only be used with native connection.
