Hi Jollybaba,
Follow these steps to get the drop down values by querying a DB using secondary data source.
1. Go to Drop-Down List box Properties
a. Select 'Look up values from an external data source' radio button under List box entries.
b. Select the secondary data source in Data source drop down.
c. For Entries field Select the column from which you want to get the values into drop down list.
2. Add a button and set a rule to query the DB.