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

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# Order By

## order by in ALIGN BY TIME mode

The result set of IoTDB is in ALIGN BY TIME mode by default and `ORDER BY TIME` clause can also be used to specify the ordering of timestamp. The SQL statement is:
```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
```

## order by in ALIGN BY DEVICE mode
When querying in ALIGN BY DEVICE mode, `ORDER BY` clause can be used to specify the ordering of result set.

ALIGN BY DEVICE mode supports four kinds of clauses with two sort keys which are `Device` and `Time`.

1. ``ORDER BY DEVICE``: sort by the alphabetical order of the device name. The devices with the same column names will be clustered in a group view.

2. ``ORDER BY TIME``: sort by the timestamp, the data points from different devices will be shuffled according to the timestamp.

3. ``ORDER BY DEVICE,TIME``: sort by the alphabetical order of the device name. The data points with the same device name will be sorted by timestamp.

4. ``ORDER BY TIME,DEVICE``: sort by timestamp. The data points with the same time will be sorted by the alphabetical order of the device name.

> To make the result set more legible, when `ORDER BY` clause is not used, default settings will be provided.
> The default ordering clause is `ORDER BY DEVICE,TIME` and the default ordering is `ASC`.

When `Device` is the main sort key, the result set is sorted by device name first, then by timestamp in the group with the same device name, the SQL statement is:
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 order by device desc,time asc align by device;
```
The result shows below:

```
+-----------------------------+-----------------+--------+------+-----------+
|                         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
```
When `Time` is the main sort key, the result set is sorted by timestamp first, then by device name in data points with the same timestamp. The SQL statement is:
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 order by time asc,device desc align by device;
```
The result shows below:
```
+-----------------------------+-----------------+--------+------+-----------+
|                         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
```
When `ORDER BY` clause is not used, sort in default way, the SQL statement is：
```sql
select * from root.ln.** where time <= 2017-11-01T00:01:00 align by device;
```
The result below indicates `ORDER BY DEVICE ASC,TIME ASC` is the clause in default situation.
`ASC` can be omitted because it's the default ordering.
```
+-----------------------------+-----------------+--------+------+-----------+
|                         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
```
Besides，`ALIGN BY DEVICE` and `ORDER BY` clauses can be used with aggregate query，the SQL statement is：
```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
```
The result shows below:
```
+-----------------------------+-----------------+---------------+-------------+------------------+
|                         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
```