# ORDER BY

> 介绍了 `ORDER BY` 语法的基本用法，通过示例展示了如何对数据进行升序或降序排序。

# ORDER BY

`ORDER BY` 语法用于根据 `SELECT` 语句中的一个或多个列对数据进行升序或降序排序。

# Syntax

`ORDER BY` 的基本语法如下：

```sql
SELECT column1, column2, ...
FROM table_name
ORDER BY column1 [ASC | DESC], column2 [ASC | DESC], ...;
```

`ORDER BY` 可以用于一个或多个列。ASC 关键字用于升序排序（默认），DESC 关键字用于降序排序。

# 示例

假如我们有一个名为 `system_metrics` 的表：

```sql
+-------+-------+----------+-------------+-----------+---------------------+
| host  | idc   | cpu_util | memory_util | disk_util | ts                  |
+-------+-------+----------+-------------+-----------+---------------------+
| host1 | idc_a |     11.8 |        10.3 |      10.3 | 2022-11-03 03:39:57 |
| host1 | idc_b |       50 |        66.7 |      40.6 | 2022-11-03 03:39:57 |
| host1 | idc_c |     50.1 |        66.8 |      40.8 | 2022-11-03 03:39:57 |
| host1 | idc_e |     NULL |        66.7 |      40.6 | 2022-11-03 03:39:57 |
| host2 | idc_a |     80.1 |        70.3 |        90 | 2022-11-03 03:39:57 |
+-------+-------+----------+-------------+-----------+---------------------+
```

要根据 `memory_util` 列对数据进行升序排序，可以使用以下 SQL 语句：

```sql
SELECT * FROM system_metrics
ORDER BY memory_util ASC;
```

结果为：

```sql
+-------+-------+----------+-------------+-----------+---------------------+
| host  | idc   | cpu_util | memory_util | disk_util | ts                  |
+-------+-------+----------+-------------+-----------+---------------------+
| host1 | idc_a |     11.8 |        10.3 |      10.3 | 2022-11-03 03:39:57 |
| host1 | idc_b |       50 |        66.7 |      40.6 | 2022-11-03 03:39:57 |
| host1 | idc_e |     NULL |        66.7 |      40.6 | 2022-11-03 03:39:57 |
| host1 | idc_c |     50.1 |        66.8 |      40.8 | 2022-11-03 03:39:57 |
| host2 | idc_a |     80.1 |        70.3 |        90 | 2022-11-03 03:39:57 |
+-------+-------+----------+-------------+-----------+---------------------+
```

要根据 `disk_util` 列对数据进行降序排序，可以使用以下 SQL 语句：

```sql
SELECT * FROM system_metrics
ORDER BY disk_util DESC;
```

结果为：

```sql
+-------+-------+----------+-------------+-----------+---------------------+
| host  | idc   | cpu_util | memory_util | disk_util | ts                  |
+-------+-------+----------+-------------+-----------+---------------------+
| host2 | idc_a |     80.1 |        70.3 |        90 | 2022-11-03 03:39:57 |
| host1 | idc_c |     50.1 |        66.8 |      40.8 | 2022-11-03 03:39:57 |
| host1 | idc_b |       50 |        66.7 |      40.6 | 2022-11-03 03:39:57 |
| host1 | idc_e |     NULL |        66.7 |      40.6 | 2022-11-03 03:39:57 |
| host1 | idc_a |     11.8 |        10.3 |      10.3 | 2022-11-03 03:39:57 |
+-------+-------+----------+-------------+-----------+---------------------+
```