Dynamic Columns Selection

 Hi All

I need to select columns dynamically in select statement. For example on the basis of few parameters select clause will select col1,col2,col3 and on other parameter values  select clause will select col3,col2,col1. I am trying to achieve this through bind parameters like SELECT :p_col1 and in AFTERPARAMETERFORM trigger I am returning column names. But in output instead of data returned string is being selected. How to resolve this?