'If..else if' conditions

I'm stuck on the 'if...else if' condition.
Let's say that i have a list of sectors and different lists of subsectors, each one exclusively reffered to one of the sectors from the first list.
Let's say that i have a table of all the clients that i'm working for.
I'd like, for each client, to specify the sector they belong to: here it's easy with the dropdown menu for example.
Then, i'd like that, once i choose one sector, in the subsector column only the proper subsectors list will appear.