237 lines
6.4 KiB
Markdown
237 lines
6.4 KiB
Markdown
---
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title: Extract scalar values in Flux
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list_title: Extract scalar values
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description: >
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Use Flux dynamic query functions to extract scalar values from Flux query output.
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This lets you, for example, dynamically set variables using query results.
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menu:
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influxdb_2_0:
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name: Extract scalar values
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parent: Query with Flux
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weight: 220
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influxdb/v2.0/tags: [scalar]
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related:
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- /{{< latest "flux" >}}/function-types/#dynamic-queries, Flux dynamic query functions
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aliases:
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- /influxdb/v2.0/query-data/guides/scalar-values/
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list_code_example: |
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```js
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scalarValue = {
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_record =
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data
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|> findRecord(fn: key => true, idx: 0)
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return _record._value
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}
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```
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---
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Use Flux [dynamic query functions](/{{< latest "flux" >}}/function-types/#dynamic-queries)
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to extract scalar values from Flux query output.
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This lets you, for example, dynamically set variables using query results.
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**To extract scalar values from output:**
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1. [Extract a column from the input stream](#extract-a-column)
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_**or**_ [extract a row from the input stream](#extract-a-row).
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2. Use the returned array or record to reference scalar values.
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_The samples on this page use the [sample data provided below](#sample-data)._
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{{% warn %}}
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#### Current limitations
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- The InfluxDB user interface (UI) does not currently support raw scalar output.
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Use [`map()`](/{{< latest "flux" >}}/stdlib/universe/map/) to add
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scalar values to output data.
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{{% /warn %}}
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## Table extraction
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Flux formats query results as a stream of tables.
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Both [`findColumn()`](/{{< latest "flux" >}}/stdlib/universe/findcolumn/)
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and [`findRecord()`](/{{< latest "flux" >}}/stdlib/universe/findrecord/)
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extract the first table in a stream of tables whose [group key](/{{< latest "flux" >}}/get-started/data-model/#group-key)
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values match the `fn` [predicate function](/{{< latest "flux" >}}/get-started/syntax-basics/#predicate-functions).
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{{% note %}}
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#### Extract the correct table
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Flux functions do not guarantee table order.
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`findColumn()` and `findRecord` extract only the **first** table that matches the `fn` predicate.
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To extract the correct table, be very specific in your predicate function or
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filter and transform your data to minimize the number of tables piped-forward into the functions.
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{{% /note %}}
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## Extract a column
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Use the [`findColumn()` function](/{{< latest "flux" >}}/stdlib/universe/findcolumn/)
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to output an array of values from a specific column in the extracted table.
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_See [Sample data](#sample-data) below._
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```js
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sampleData
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|> findColumn(
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fn: (key) => key._field == "temp" and key.location == "sfo",
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column: "_value"
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)
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// Returns [65.1, 66.2, 66.3, 66.8]
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```
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### Use extracted column values
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Use a variable to store the array of values.
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In the example below, `SFOTemps` represents the array of values.
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Reference a specific index (integer starting from `0`) in the array to return the
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value at that index.
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_See [Sample data](#sample-data) below._
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```js
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SFOTemps = sampleData
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|> findColumn(
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fn: (key) => key._field == "temp" and key.location == "sfo",
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column: "_value"
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)
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SFOTemps
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// Returns [65.1, 66.2, 66.3, 66.8]
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SFOTemps[0]
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// Returns 65.1
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SFOTemps[2]
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// Returns 66.3
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```
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## Extract a row
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Use the [`findRecord()` function](/{{< latest "flux" >}}/stdlib/universe/findrecord/)
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to output data from a single row in the extracted table.
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Specify the index of the row to output using the `idx` parameter.
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The function outputs a record with key-value pairs for each column.
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```js
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sampleData
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|> findRecord(
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fn: (key) => key._field == "temp" and key.location == "sfo",
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idx: 0
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)
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// Returns {
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// _time:2019-11-11T12:00:00Z,
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// _field:"temp",
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// location:"sfo",
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// _value: 65.1
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// }
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```
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### Use an extracted row record
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Use a variable to store the extracted row record.
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In the example below, `tempInfo` represents the extracted row.
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Use [dot or bracket notation](/{{< latest "flux" >}}/data-types/composite/record/#dot-notation)
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to reference keys in the record.
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```js
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tempInfo = sampleData
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|> findRecord(
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fn: (key) => key._field == "temp" and key.location == "sfo",
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idx: 0
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)
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tempInfo
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// Returns {
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// _time:2019-11-11T12:00:00Z,
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// _field:"temp",
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// location:"sfo",
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// _value: 65.1
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// }
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tempInfo._time
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// Returns 2019-11-11T12:00:00Z
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tempInfo.location
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// Returns sfo
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```
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## Example helper functions
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Create custom helper functions to extract scalar values from query output.
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##### Extract a scalar field value
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```js
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// Define a helper function to extract field values
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getFieldValue = (tables=<-, field) => {
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extract = tables
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|> findColumn(
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fn: (key) => key._field == field,
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column: "_value"
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)
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return extract[0]
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}
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// Use the helper function to define a variable
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lastJFKTemp = sampleData
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|> filter(fn: (r) => r.location == "kjfk")
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|> last()
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|> getFieldValue(field: "temp")
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lastJFKTemp
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// Returns 71.2
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```
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##### Extract scalar row data
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```js
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// Define a helper function to extract a row as a record
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getRow = (tables=<-, field, idx=0) => {
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extract = tables
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|> findRecord(
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fn: (key) => true,
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idx: idx
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)
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return extract
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}
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// Use the helper function to define a variable
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lastReported = sampleData
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|> last()
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|> getRow(field: "temp")
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"The last location to report was ${lastReported.location}.
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The temperature was ${string(v: lastReported._value)}°F."
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// Returns:
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// The last location to report was kord.
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// The temperature was 38.9°F.
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```
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---
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## Sample data
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The following sample data set represents fictional temperature metrics collected
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from three locations.
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It's formatted in [annotated CSV](/influxdb/v2.0/reference/syntax/annotated-csv/) and imported
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into the Flux query using the [`csv.from()` function](/{{< latest "flux" >}}/stdlib/csv/from/).
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Place the following at the beginning of your query to use the sample data:
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{{% truncate %}}
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```js
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import "csv"
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sampleData = csv.from(csv: "
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#datatype,string,long,dateTime:RFC3339,string,string,double
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#group,false,true,false,true,true,false
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#default,,,,,,
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,result,table,_time,location,_field,_value
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,,0,2019-11-01T12:00:00Z,sfo,temp,65.1
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,,0,2019-11-01T13:00:00Z,sfo,temp,66.2
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,,0,2019-11-01T14:00:00Z,sfo,temp,66.3
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,,0,2019-11-01T15:00:00Z,sfo,temp,66.8
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,,1,2019-11-01T12:00:00Z,kjfk,temp,69.4
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,,1,2019-11-01T13:00:00Z,kjfk,temp,69.9
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,,1,2019-11-01T14:00:00Z,kjfk,temp,71.0
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,,1,2019-11-01T15:00:00Z,kjfk,temp,71.2
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,,2,2019-11-01T12:00:00Z,kord,temp,46.4
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,,2,2019-11-01T13:00:00Z,kord,temp,46.3
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,,2,2019-11-01T14:00:00Z,kord,temp,42.7
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,,2,2019-11-01T15:00:00Z,kord,temp,38.9
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")
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```
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{{% /truncate %}}
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