selecting Null in parameter list

For oracle 6i

My parameter query is

select distinct sourcetext from table_all union select * from dual

Which shows a blank or Null selection with the other values

This is in my query where clause

   A.sourcetext like (decode(:P_sourcetext,'*','%',:P_sourcetext))

How to I fix the where clause so that if null values are present in the parameter list, when the user selects the Null value from the drop down list or
selects * (to get all the values) the report will show null values. Received on Wed Nov 15 2000 - 19:53:54 CST