Use the `GROUP BY` clause to group data by values.

`GROUP BY` is an optional clause used to group rows that have the same values for all columns and expressions in the list.
To output an aggregation for each group, include an aggregate or selector function in the `SELECT` statement.
When `GROUP BY` appears in a query, the `SELECT` list can only use columns that appear in the `GROUP BY` list
or in aggregate expressions.

`GROUP BY` can use column aliases that are defined in the `SELECT` clause.
`GROUP BY` can't use an alias named `time`.
In a `GROUP BY` list, `time` always refers to the measurement `time` column.

- [Syntax](#syntax)
- [Examples](#examples)

## Syntax

```sql
SELECT
  AGGREGATE_FN(field1),
  tag1
FROM measurement
GROUP BY tag1
```

## Examples

### Group data by tag values

```sql
SELECT
  AVG("water_level") AS "avg_water_level",
  "location"
FROM "h2o_feet" 
GROUP BY "location"
```

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|   avg_water_level | location     |
| ----------------: | ------------ |
| 5.359142420303919 | coyote_creek |
| 3.530712094245885 | santa_monica |

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Group results in 15 minute time intervals by tag:

```sql
SELECT
  "location",
  DATE_BIN(INTERVAL '15 minutes', time, TIMESTAMP '2022-01-01 00:00:00Z') AS _time,
  COUNT("water_level") AS count
FROM "h2o_feet"
WHERE 
  time >= timestamp '2019-09-17T00:00:00Z'
  AND time <= timestamp '2019-09-17T01:00:00Z'
GROUP BY
  _time,
  location
ORDER BY
  location,
  _time
```

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The query uses the `COUNT()` function to count the number of `water_level` points per 15 minute interval.
Results are then ordered by location and time.

| location     | _time                 | count |
| :----------- | :-------------------- | ----: |
| coyote_creek | 2019-09-16T23:45:00Z  |     1 |
| coyote_creek | 2019-09-17T00:00:00Z  |     2 |
| coyote_creek | 2019-09-17T00:15:00Z  |     3 |
| coyote_creek | 2019-09-17T00:30:00Z  |     2 |
| coyote_creek | 2019-09-17T00:45:00Z  |     3 |
| santa_monica | 2019-09-16T23:45:00Z  |     1 |
| santa_monica | 2019-09-17T00:00:00Z  |     2 |
| santa_monica | 2019-09-17T00:15:00Z  |     3 |
| santa_monica | 2019-09-17T00:30:00Z  |     2 |
| santa_monica | 2019-09-17T00:45:00Z  |     3 |

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