Need help with programming a column based on previous data in the same column
I have two columns section and subsection. When I add new books, I first fill in the section (e.g. 'psychology', geography etc.). While entering new books, when I type a letter, I get all the previously entered unique data, which is great. I then proceed to fill in the subsection. I get all the previously entered data in this column, BUT I want only a subset of data that belongs to a particular section. For example, if under 'psychology', I have two subsections - 'Abnormal' and 'Clinical', and under 'geography', I have two subsections - 'Africa' and 'Asia'; then if I type 'Psychology' as the section for a new book and in the subsection I type 'A', I wish to see only 'Abnormal' and not 'Africa' and 'Asia'.
So I want the column 'subsection' to have data based on column 'section'.
Before someone points out, yes, using tags is a great idea, but I feel that I have better control here.