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Recalculate the _value column

This example uses NOAA water sample data.

Recalculate the _value column without creating a new one. Use the with operator in map() to overwrite the existing _value column.

The following query:

  • Uses filter() to filter the average_temperature measurement.
  • Uses map() to convert Fahrenheit temperature values into Celsius.

from(bucket: "noaa")
  |> filter(fn: (r) => r._measurement == "average_temperature")
  |> range(start: -30d)
  |> map(fn: (r) => ({r with _value: (float(v: r._value) - 32.0) * 5.0 / 9.0} ))
_field _measurement _start _stop _time location _value
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:00:00Z coyote_creek 27.77777777777778
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:06:00Z coyote_creek 22.77777777777778
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:12:00Z coyote_creek 30
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:18:00Z coyote_creek 31.666666666666668
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:24:00Z coyote_creek 25
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:30:00Z coyote_creek 21.11111111111111
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:36:00Z coyote_creek 28.88888888888889
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:42:00Z coyote_creek 24.444444444444443
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:48:00Z coyote_creek 29.444444444444443
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T00:54:00Z coyote_creek 26.666666666666668
degrees average_temperature 1920-03-05T22:10:01Z 2020-03-05T22:10:01Z 2019-08-17T01:00:00Z coyote_creek 21.11111111111111
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