Skip to main content
2 of 11
added 128 characters in body
Solomon Rutzky
  • 69.5k
  • 8
  • 155
  • 300

The proper structure for this scenario is a SubClass / Inheritance model, and is nearly identical to the concept I proposed in this answer: Heterogeneous ordered list of value.

The model proposed in this question is actually quite close in that the Animal entity contains the type (i.e. race) and the properties that are common across all types. However, there are two minor changes that are needed:

  1. Remove the Cat_ID and Dog_ID fields from their respective entities:

The key concept here is that everything is an Animal, regardless of race: Cat, Dog, Elephant, and so on. Given that starting point, any particular race of Animal doesn't truly need a separate identifier since:

  1. the Animal_ID is unique
  2. the Cat, Dog, and any additional race entities added in the future do not, by themselves, fully represent any particular Animal; they only have meaning when used in combination with the information contained in the parent entity, Animal.

Hence, the Animal_ID property in the Cat, Dog, etc entities is both the PK and the FK back to the Animal entity.

  1. Differentiate between types of breed:

Just because two properties share the same name does not necessarily mean that those properties are the same, even if the name being the same implies such a relationship. In this case, what you really have is actually CatBreed and DogBreed as seperate "types"

{will update shortly with the appropriate SQL}

Additional Notes:

  1. The concept of breed seems to be a focal point for confusion. It was suggested by jcolebrand (in a comment on the question) that breed is a property shared across the different races, and the other two answers have it integrated as such in their models. This is a mistake, however, because the values for breed are not shared across the different values of race. Yes, I am aware that the two other proposed models attempt to solve this issue by making race a parent of breed. While that technically solves the relationship issue, it doesn't help solve the overall modeling question of what to do about non-common properties, nor how to handle a race that does not have a breed. But, in the case that such a property were guaranteed to exist across all Animals, I will include an option for that as well.
  2. The models proposed by vijayp and DavidN (which seem to be identical) do not work because:
  3. They either
    1. do not allow for non-common properties to be stored (at least not for individual instances of any Animal), or
    2. require that all properties for all races be stored in the Animal entity which is a very flat (and nearly non-relational) way of representing this data. Yes, people do this all of the time, but it means having many NULL fields per row for the properties that are not meant for that particular race AND knowing which fields per row are associated with the particular race of that record.
  4. They do not allow for adding a race of Animal in the future that does not have breed as a property. And even if ALL Animals have a breed, that wouldn't change the structure due to what has been previously noted about breed: that breed is dependent on the race (i.e. breed for Cat is not the same thing as breed for Dog).
Solomon Rutzky
  • 69.5k
  • 8
  • 155
  • 300