SSRS - Dataset refreshes with wrong fields

Hello,

 I have a SSRS report containing two data sets which calls a stored procedure to get the data.
 The Stored procedure code is something like below:

USE AdventureWorks
GO

CREATE PROCEDURE dbo.usp_get_data
(@output VARCHAR(50))
AS
BEGIN

	IF @Output = 'details'
	BEGIN
		   SELECT * FROM AdventureWorks.dbo.details;
	END

	IF @Output = 'summary'
	BEGIN
		 SELECT * FROM AdventureWorks.dbo.Summary;
	END
END

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In SSRS, one data set is for summary and another one is for details.
In the data set,  I have hardcode the @output = 'details' for data set 1, and hardcoded @output = 'summary' for data set 2.

When I refresh data set 2 which is for summary, SSRS refreshes the dataset with details fields instead of the summary fields. I think it is doing that because details IF block is the first one in the code.

Can you please help me in fixing this issue and get SSRS wit refresh with correct list of fields?

Thanks!