Re: How would you choose keys for the following?

[James Gordon]

I would not have owner in the diseases table as it is the pet that has the disease not the owner.

Personally I would go for a many-to-many joing between pets and diseases.

owners
~~~~~~
ownerid (auto increment)
Name
...
|
|
/|\
Pets
~~~~
petid (auto increment)
Name
...
|
/|\
petdiseases
~~~~~~~~~~~
petid
diseaseid
medication_taken
...
\|/
|
diseases
~~~~~~~~
diseaseid (auto increment)
Description
...