docs/zh/UserGuide/Query-Data/Order-By.md

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## 结果集排序

### 时间对齐模式下的排序
IoTDB的查询结果集默认按照时间对齐，可以使用`ORDER BY TIME`的子句指定时间戳的排列顺序。示例代码如下：
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 order by time desc;
```
执行结果：

```
+-----------------------------+--------------------------+------------------------+-----------------------------+------------------------+
|                         Time|root.ln.wf02.wt02.hardware|root.ln.wf02.wt02.status|root.ln.wf01.wt01.temperature|root.ln.wf01.wt01.status|
+-----------------------------+--------------------------+------------------------+-----------------------------+------------------------+
|2017-11-01T00:01:00.000+08:00|                        v2|                    true|                        24.36|                    true|
|2017-11-01T00:00:00.000+08:00|                        v2|                    true|                        25.96|                    true|
|1970-01-01T08:00:00.002+08:00|                        v2|                   false|                         null|                    null|
|1970-01-01T08:00:00.001+08:00|                        v1|                    true|                         null|                    null|
+-----------------------------+--------------------------+------------------------+-----------------------------+------------------------+
Total line number = 4
```
### 设备对齐模式下的排序
当使用`ALIGN BY DEVICE`查询对齐模式下的结果集时，可以使用`ORDER BY`子句对返回的结果集顺序进行规定。

在设备对齐模式下支持4种排序模式的子句,其中包括两种排序键，`DEVICE`和`TIME`，靠前的排序键为主排序键，每种排序键都支持`ASC`和`DESC`两种排列顺序。
1. ``ORDER BY DEVICE``: 按照设备名的字典序进行排序，排序方式为字典序排序，在这种情况下，相同名的设备会以组的形式进行展示。

2. ``ORDER BY TIME``: 按照时间戳进行排序，此时不同的设备对应的数据点会按照时间戳的优先级被打乱排序。

3. ``ORDER BY DEVICE,TIME``: 按照设备名的字典序进行排序，设备名相同的数据点会通过时间戳进行排序。

4. ``ORDER BY TIME,DEVICE``: 按照时间戳进行排序，时间戳相同的数据点会通过设备名的字典序进行排序。

> 为了保证结果的可观性，当不使用`ORDER BY`子句，仅使用`ALIGN BY DEVICE`时，会为设备视图提供默认的排序方式。其中默认的排序视图为``ORDER BY DEVCE,TIME``，默认的排序顺序为`ASC`，
> 即结果集默认先按照设备名升序排列，在相同设备名内再按照时间戳升序排序。


当主排序键为`DEVICE`时，结果集的格式与默认情况类似：先按照设备名对结果进行排列，在相同的设备名下内按照时间戳进行排序。示例代码如下：
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 order by device desc,time asc align by device;
```
执行结果：

```
+-----------------------------+-----------------+--------+------+-----------+
|                         Time|           Device|hardware|status|temperature|
+-----------------------------+-----------------+--------+------+-----------+
|1970-01-01T08:00:00.001+08:00|root.ln.wf02.wt02|      v1|  true|       null|
|1970-01-01T08:00:00.002+08:00|root.ln.wf02.wt02|      v2| false|       null|
|2017-11-01T00:00:00.000+08:00|root.ln.wf02.wt02|      v2|  true|       null|
|2017-11-01T00:01:00.000+08:00|root.ln.wf02.wt02|      v2|  true|       null|
|2017-11-01T00:00:00.000+08:00|root.ln.wf01.wt01|    null|  true|      25.96|
|2017-11-01T00:01:00.000+08:00|root.ln.wf01.wt01|    null|  true|      24.36|
+-----------------------------+-----------------+--------+------+-----------+
Total line number = 6
```
主排序键为`Time`时，结果集会先按照时间戳进行排序，在时间戳相等时按照设备名排序。
示例代码如下：
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 order by time asc,device desc align by device;
```
执行结果：
```
+-----------------------------+-----------------+--------+------+-----------+
|                         Time|           Device|hardware|status|temperature|
+-----------------------------+-----------------+--------+------+-----------+
|1970-01-01T08:00:00.001+08:00|root.ln.wf02.wt02|      v1|  true|       null|
|1970-01-01T08:00:00.002+08:00|root.ln.wf02.wt02|      v2| false|       null|
|2017-11-01T00:00:00.000+08:00|root.ln.wf02.wt02|      v2|  true|       null|
|2017-11-01T00:00:00.000+08:00|root.ln.wf01.wt01|    null|  true|      25.96|
|2017-11-01T00:01:00.000+08:00|root.ln.wf02.wt02|      v2|  true|       null|
|2017-11-01T00:01:00.000+08:00|root.ln.wf01.wt01|    null|  true|      24.36|
+-----------------------------+-----------------+--------+------+-----------+
Total line number = 6
```
当没有显式指定时，主排序键默认为`Device`，排序顺序默认为`ASC`，示例代码如下：
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 align by device;
```
结果如图所示，可以看出，`ORDER BY DEVICE ASC,TIME ASC`就是默认情况下的排序方式，由于`ASC`是默认排序顺序，此处可以省略。
```
+-----------------------------+-----------------+--------+------+-----------+
|                         Time|           Device|hardware|status|temperature|
+-----------------------------+-----------------+--------+------+-----------+
|2017-11-01T00:00:00.000+08:00|root.ln.wf01.wt01|    null|  true|      25.96|
|2017-11-01T00:01:00.000+08:00|root.ln.wf01.wt01|    null|  true|      24.36|
|1970-01-01T08:00:00.001+08:00|root.ln.wf02.wt02|      v1|  true|       null|
|1970-01-01T08:00:00.002+08:00|root.ln.wf02.wt02|      v2| false|       null|
|2017-11-01T00:00:00.000+08:00|root.ln.wf02.wt02|      v2|  true|       null|
|2017-11-01T00:01:00.000+08:00|root.ln.wf02.wt02|      v2|  true|       null|
+-----------------------------+-----------------+--------+------+-----------+
Total line number = 6
```
同样，可以在聚合查询中使用`ALIGN BY DEVICE`和`ORDER BY`子句，对聚合后的结果进行排序，示例代码如下所示：
```sql
select count(*) from root.ln.** group by ((2017-11-01T00:00:00.000+08:00,2017-11-01T00:03:00.000+08:00],1m) order by device asc,time asc align by device
```
执行结果：
```
+-----------------------------+-----------------+---------------+-------------+------------------+
|                         Time|           Device|count(hardware)|count(status)|count(temperature)|
+-----------------------------+-----------------+---------------+-------------+------------------+
|2017-11-01T00:01:00.000+08:00|root.ln.wf01.wt01|           null|            1|                 1|
|2017-11-01T00:02:00.000+08:00|root.ln.wf01.wt01|           null|            0|                 0|
|2017-11-01T00:03:00.000+08:00|root.ln.wf01.wt01|           null|            0|                 0|
|2017-11-01T00:01:00.000+08:00|root.ln.wf02.wt02|              1|            1|              null|
|2017-11-01T00:02:00.000+08:00|root.ln.wf02.wt02|              0|            0|              null|
|2017-11-01T00:03:00.000+08:00|root.ln.wf02.wt02|              0|            0|              null|
+-----------------------------+-----------------+---------------+-------------+------------------+
Total line number = 6
```