While performing joins you need specific columns of the joined tables so for doing that you need to mention the name of the column with their table name.

This task gets tedious because you have to write your table name with every column you want.

So a better way would be to create table alias while performing joins it will reduce your effort by substituting a big table name with a smaller one.

ALTER PROC [dbo].[uspGetNudgesDetails]
@AccessToken VARCHAR(50),
@BeaconID VARCHAR(50)=NULL
AS

BEGIN

	DECLARE @CompanyID BIGINT =0
	DECLARE @TargetUserID BIGINT=0
	DECLARE @UserName varchar(50)
-- Get user and company

	SELECT @UserName=[FirstName] + ' ' + LastName, 
			@TargetUserID=[ID],
			@CompanyID=[CompanyID]
		FROM [User] WHERE AccessToken=@AccessToken


		----DECLARE @CreatedDate AS DATETIME = GETDATE()
		----DECLARE @ExpiryDate AS DATETIME = (SELECT DATEADD(DAY, @DaysTillExpire, @CreatedDate))
		----SET @ExpiryDate = (SELECT DATEADD(HOUR, @HoursTillExpire, @ExpiryDate))

		UPDATE NUDGE
		SET DaysTillExpire = CASE  WHEN DATEDIFF(day, GETDATE(), ExpiryDate) < 0 THEN 0
									ELSE DATEDIFF(day, GETDATE(), ExpiryDate)
							END				
		,
		HoursTillExpire = CASE  WHEN DATEDIFF( hour, GETDATE(), ExpiryDate) < 0 THEN 0
									ELSE DATEDIFF(day, GETDATE(), ExpiryDate)
							END				

		,
		IsExpired = CASE WHEN DATEDIFF(day, GETDATE(), ExpiryDate) < 0 AND DATEDIFF( hour, GETDATE(), ExpiryDate) < 0 THEN 1
							ELSE 0
							END
-- For general message with no location

	SELECT N.[ID],
		N.[Message],
		@BeaconID as [BeaconID],		
		N.[LocationID],
		NULL AS Location,
		N.[TargetUserID],
		@UserName as [TargetUser],	
		N.[IsCompleted],
		N.[IsDelivered],
		N.[IsExpired],
		N.[IsDeleted],
		N.[Url],		
		N.[DaysTillExpire],
		N.[HoursTillExpire],
		--DATEDIFF( day, GETDATE(), N.ExpiryDate ) AS DaysTillExpire,
		--DATEDIFF( hour, GETDATE(), N.ExpiryDate ) AS HoursTillExpire,
		(U.[FirstName] + ' ' + U.[LastName]) AS [CreatedBy],
		CONVERT(VARCHAR(20), N.[CreatedDate], 103) as [CreatedDate],
		@CompanyID as [CompanyID]
		
		FROM [NUDGE] AS N 
				INNER JOIN  [User] AS U ON N.[CreatedBy]=U.[ID] 
		WHERE N.[LocationID]=0 
				AND N.[TargetUserID]=@TargetUserID
				AND N.[IsDeleted]=0 
				AND N.[IsExpired]=0 
				AND N.[IsCompleted]=0 

	UNION

-- For location dependent messages

	SELECT  N.[ID],
		N.[Message],
		@BeaconID as [BeaconID],
		N.[LocationID],
		L.[Location],
		N.[TargetUserID],
		@UserName as [TargetUser],	
		N.[IsCompleted],
		N.[IsDelivered],
		N.[IsExpired],
		N.[IsDeleted],		
		N.[Url],		
		N.[DaysTillExpire],
		N.[HoursTillExpire],
		--DATEDIFF( day, GETDATE(), N.ExpiryDate ) AS DaysTillExpire,
		--DATEDIFF( hour, GETDATE(), N.ExpiryDate ) AS HoursTillExpire,	
		(U.[FirstName] + ' ' + U.[LastName]) AS [CreatedBy],
		CONVERT(VARCHAR(20), N.[CreatedDate], 103) as [CreatedDate],
		@CompanyID as [CompanyID]
		
		
		FROM [NUDGE] AS N 
			INNER JOIN  [User] AS U ON N.[CreatedBy]=U.[ID] 
			INNER JOIN [LOCATION] AS L ON L.[ID]=N.[LocationID] 
		WHERE N.[LocationID]=(SELECT [LocationID] FROM [Location] WHERE [BeaconUUID]=@BeaconID and [CompanyID]=@CompanyID)
			AND N.[TargetUserID]=@TargetUserID
			AND N.[IsDeleted]=0 
			AND N.[IsExpired]=0 
			AND N.[IsCompleted]=0 
END

Here while making joins we are using table alias like for User we are using U and for Location we are using L

This makes our query writing easy and effective in terms of execution