How do I design a database to store changes over time?

Question

This database will store a list of children. But the problem is, they will have their weight measured once a day. How can I store the changes so I can easily query their actual weight and the weight variation over one day, one week and one month?

Answer 1

I'd think something like the following:

table kid
    int pkey(id)
    text name

table weight
    date when
    int kidid fkey(kid.id)
    int weight
    int pkey(id)