Hi Fione,
1) Create a new command object and type in the SQL that would return just the List of Values. E.g:
Select distinct table.field from table
2) DO NOT Join this command object with the other command object
3) Ignore the warning.
4) Edit the prompt from the Field Explorer > Set it to 'Dynamic' > From the 'Value' options choose the field from the Command Object you created in Step 1
-Abhilash