If my data looks like this:

Category    Sub Category
1           a
            b
2           c
            d

Which shows that there are two categories: "1", which has sub categories "a" and "b", and "2", which has sub categories "c" and "d".

What can I do in excel (for filtering/sorting) to keep rows 1 and 2 together as category "1", instead of the first row as category "1", and the second as category ""?

I'm trying to avoid having to do this:

Category    Sub Category
1           a
1           b
2           c
2           d