How to get query parameters in a Logic App?

Question

I\'m trying to pass an extra query parameter to Azure logic app so that I can process below data in the Logic App workflow

For Example https://logicURL?SelectedData=

Answer 1

Context

• MSFT Azure Logicapp
• MSFT Logicapp workflow definition language
• Live version as of 2020-06-25 04:56:31

Problem

• Logicapp developer wants to obtain the value of a URL query parameter passed in via HTTP GET

Solution

• The solution to this use-case is already provided elsewhere in this StackOverflow thread
• This addon answer, however, refactors the previous solution
  • it addresses constructing expressions in MSFT Workflow definition language (the JSON-based source code you see when viewing a logicapp in "code" view)

Details

• This URL extends the original question and another answer in this SO Thread
• Here we expect FirstName LastName and FaveColor properties

    https://xyz.logic.azure.com:443/workflows/id/triggers/manual/paths/invoke
      ?api-version=2016-10-01
      &sp=%2Ftriggers%2Fmanual%2Frun
      &sv=1.0
      &sig=code
      &FirstName=Huomer
      &LastName=Huimpson
      &FaveColor=     

• Standard init: The following is sufficient to capture the desired name-value pairs

triggerOutputs()['queries']['FirstName']
triggerOutputs()['queries']['LastName']
triggerOutputs()['queries']['FaveColor']

• Error-trap init: The following is sufficient to capture the desired name-value pairs without throwing an error if any desired name-value pair is missing (error-free capture)

triggerOutputs()['queries']?['FirstName']
triggerOutputs()['queries']?['LastName']
triggerOutputs()['queries']?['FaveColor']

• Error-trap init with defaults: The following is sufficient to error-trap init the desired name-value pairs, as well as provide a default value for any missing values

coalesce(triggerOutputs()['queries']?['FirstName']  , 'Puomer'  )
coalesce(triggerOutputs()['queries']?['LastName']   , 'Puimpson' )
coalesce(triggerOutputs()['queries']?['FaveColor']  , 'Purple' )

Solution refactored

• Consequently, the original solution can be refactored as follows

@{coalesce(triggerOutputs()['queries']?['SelectedData'] , '__blank__')}

• This approach does what the typical use-case calls for, which is:
  • get the value if it exists,
  • otherwise provide a default value, and
  • don't crash the entire logicapp if the parameter was completely omitted
  • the @{} syntax can be used if you are editing workflow definition language directly, but not if you are entering it in the "expression dialog box"

See also

• https://stackoverflow.com/a/51058306/42223
• How to do a 'null' check in 'if' condition action of Azure Logic App
• https://docs.microsoft.com/en-us/azure/logic-apps/workflow-definition-language-functions-reference#coalesce