Write conditional INNER JOIN in SQL Server Query

Dynamic SQL // :

DECLARE @Query1 varchar(max),@Query2 varchar(max), @Variable varchar(20),@Flag varchar(20)
SET @Variable = '''TEST'''
SET @Flag = 'asd'
SET @Query1 = 'SELECT * FROM DATA WHERE VOUCHERNO ='+@Variable
SET @Query2 = 'SELECT * FROM DATA D INNER JOIN MR000 M ON D.BookNo = M.MasterID WHERE VOUCHERNO ='+@Variable

BEGIN
IF(@Flag<>@Variable)
	BEGIN
		EXEC(@Query2)
	END
ELSE
	BEGIN 
		EXEC(@Query1)
	END
END

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