Before using the database custom field of the Select List type, ensure that the type of the custom field is "Select List" on the Configuring Power Database Custom Fields page.
With the configuration as in the screenshot above, the Product Name database custom field will now display values in a combo box on the create, edit, or transition screen of an issue.
Select list displays options with the values returned executing the query. When you pick an option for the product name, and the value of the child field Product Id will be set automatically.
If you select "None" for the Product Name (the default provided value), then the value of the child database custom field (Product Id in our case) will be cleared.
After you save the ticket, the values for the custom fields are displayed in the view issue screen.
Adding "None" option
Starting with version 1.1.4 / 2.1.4 of the app, you can define whether to show the "None" option for each individual field by selecting the Add None Option checkbox in the field configuration.