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# 查询对齐模式

在 IoTDB 中，查询结果集**默认按照时间对齐**，包含一列时间列和若干个值列，每一行数据各列的时间戳相同。

除按照时间对齐外，还支持以下对齐模式：

- 按设备对齐 `ALIGN BY DEVICE`

## 按设备对齐

在按设备对齐模式下，设备名会单独作为一列出现，查询结果集包含一列时间列、一列设备列和若干个值列。如果 `SELECT` 子句中选择了 `N` 列，则结果集包含 `N + 2` 列（时间列和设备名字列）。

在默认情况下，结果集按照 `Device` 进行排列，在每个 `Device` 内按照 `Time` 列升序排序。

当查询多个设备时，要求设备之间同名的列数据类型相同。

为便于理解，可以按照关系模型进行对应。设备可以视为关系模型中的表，选择的列可以视为表中的列，`Time + Device` 看做其主键。

**示例：**

```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 align by device;
```

执行如下：

```
+-----------------------------+-----------------+-----------+------+--------+
|                         Time|           Device|temperature|status|hardware|
+-----------------------------+-----------------+-----------+------+--------+
|2017-11-01T00:00:00.000+08:00|root.ln.wf01.wt01|      25.96|  true|    null|
|2017-11-01T00:01:00.000+08:00|root.ln.wf01.wt01|      24.36|  true|    null|
|1970-01-01T08:00:00.001+08:00|root.ln.wf02.wt02|       null|  true|      v1|
|1970-01-01T08:00:00.002+08:00|root.ln.wf02.wt02|       null| false|      v2|
|2017-11-01T00:00:00.000+08:00|root.ln.wf02.wt02|       null|  true|      v2|
|2017-11-01T00:01:00.000+08:00|root.ln.wf02.wt02|       null|  true|      v2|
+-----------------------------+-----------------+-----------+------+--------+
Total line number = 6
It costs 0.012s
```