You can set up a conditional dropdown setup as follows:
Tables:
- Companies
- Sectors
- Contacts
Relationships:
- 1 Company has many Sectors
- 1 Sector has many Contacts
- 1 Company has many Contacts
To set up the conditional dropdown, go to the Contacts table:
1) Edit the field [Related Sector] - which is the reference field for the Sectors -> Contacts relationship
2) Go to Reference field options => Conditional values
3) Check the checkbox for "The values in this field depend on a selection in another field"
A selection in "Contacts: Related Company"
Shows choices where
"Contacts: Related Company" = "Sectors: Related Company"
Now, when you add a Contact, first you will select a Company, and then only the Sectors for that Company will be available for you to select.
Hope this helps!