docs-v2/content/v2.0/query-data/guides/conditional-logic.md

title	seotitle	description	v2.0/tags	menu	weight
Query using conditional logic	Query using conditional logic in Flux	This guide describes how to use Flux conditional expressions, such as `if`, `else`, and `then`, to query and transform data.	math flux	v2_0 name parent Query using conditionals How-to guides	209

Flux provides if, then, and else conditional expressions that allow for powerful and flexible Flux queries.

Conditional expression syntax

// Pattern
if <condition> then <action> else <alternative-action>

// Example
if color == "green" then "008000" else "ffffff"

Conditional expressions are most useful in the following contexts:

• When defining variables.
• When using functions that operate on a single row at a time ( filter(), map(), reduce() ).

Examples

• Conditionally set the value of a variable
• Create conditional filters
• Conditionally transform column values with map()
• Conditionally increment a count with reduce()

Conditionally set the value of a variable

The following example sets the overdue variable based on the dueDate variable's relation to now().

dueDate = 2019-05-01
overdue = if dueDate < now() then true else false

Create conditional filters

The following example uses an example metric dashboard variable to change how the query filters data. metric has three possible values:

• Memory
• CPU
• Disk

from(bucket: "example-bucket")
  |> range(start: -1h)
  |> filter(fn: (r) =>
      if v.metric == "Memory"
        then r._measurement == "mem" and r._field == "used_percent"
      else if v.metric == "CPU"
        then r._measurement == "cpu" and r._field == "usage_user"
      else if v.metric == "Disk"
        then r._measurement == "disk" and r._field == "used_percent"
      else r._measurement != ""
  )

Conditionally transform column values with map()

The following example uses the map() function to conditionally transform column values. It sets the level column to a specific string based on _value column.

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from(bucket: "example-bucket")
  |> range(start: -5m)
  |> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
  |> map(fn: (r) => ({
    _time: r._time,
    _value: r._value,
    level:
      if r._value >= 95.0000001 and r._value <= 100.0 then "critical"
      else if r._value >= 85.0000001 and r._value <= 95.0 then "warning"
      else if r._value >= 70.0000001 and r._value <= 85.0 then "high"
      else "normal"
    })
  )

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from(bucket: "example-bucket")
  |> range(start: -5m)
  |> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
  |> map(fn: (r) => ({
    // Retain the _time column in the mapped row
    _time: r._time,
    // Retain the _value column in the mapped row
    _value: r._value,
    // Set the level column value based on the _value column
    level:
      if r._value >= 95.0000001 and r._value <= 100.0 then "critical"
      else if r._value >= 85.0000001 and r._value <= 95.0 then "warning"
      else if r._value >= 70.0000001 and r._value <= 85.0 then "high"
      else "normal"
    })
  )

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Conditionally increment a count with reduce()

The following example uses the aggregateWindow() and reduce() functions to count the number of records in every five minute window that exceed a defined threshold.

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threshold = 65.0

data = from(bucket: "example-bucket")
  |> range(start: -1h)
  |> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
  |> aggregateWindow(
      every: 5m,
      fn: (column, tables=<-) => tables |> reduce(
            identity: {above_threshold_count: 0.0},
            fn: (r, accumulator) => ({
              above_threshold_count:
                if r._value >= threshold then accumulator.above_threshold_count + 1.0
                else accumulator.above_threshold_count + 0.0
            })
        )
    )

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threshold = 65.0

data = from(bucket: "example-bucket")
  |> range(start: -1h)
  |> filter(fn: (r) => r._measurement == "mem" and r._field == "used_percent" )
  // Aggregate data into 5 minute windows using a custom reduce() function
  |> aggregateWindow(
      every: 5m,
      // Use a custom function in the fn parameter.
      // The aggregateWindow fn parameter requires 'column' and 'tables' parameters.
      fn: (column, tables=<-) => tables |> reduce(
            identity: {above_threshold_count: 0.0},
            fn: (r, accumulator) => ({
              // Conditionally increment above_threshold_count if
              // r.value exceeds the threshold
              above_threshold_count:
                if r._value >= threshold then accumulator.above_threshold_count + 1.0
                else accumulator.above_threshold_count + 0.0
            })
        )
    )

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