Drop Down List Based on Previous Column's Drop Down List Selection - Categories/Sub-Categories

Any help is appreciated on the following.  Thanks!

I'm setting up a sheet with columns to categorize/sub-categorize data.

I'd like a Category Column which has a Drop-Down List and a Sub-Category Column with a Drop-Down List based on the selected "Category" from the previous Drop-Down List.

Basically, if my lists look as follows below (Category List/Sub-Category List), how do I display the Sub-Category Drop-Down List for the corresponding Category (only) without having to include *all* Sub-Categories to choose from?

• If "Category Column" Drop-Down Selection = Policy, then
  • "Sub-Category Column" Drop-Down Selection = only the option of....
    • "Personnel, Product, Refund/Cancellation, Safety, Security, Payroll, Attendance"

Categories Drop-Down List:

• Promotions Category
  • Sub-Categories for Promotions Category:
    • Corporate
    • Membership Based
    • Customer Type
    • Sale
• Policy Category
  • Sub-Categories for Policy Category:
    • Personnel
    • Product
    • Refund/Cancellation
    • Safety
    • Security
    • Payroll
    • Attendance
• System Category
  • Sub-Cateogories for System Category:
    • Update
    • Enhancement
    • Replacement
    • Alert
• Process Category
  • Sub-Categories for Process Category:
    • Call Handling
    • Product Sales
    • Requests
    • Complaints
    • Issuing Compensation