Unions

Union types share similarities with interfaces; however, they lack the ability to define any shared fields among the constituent types.

union SearchResult = Human | Droid | Starship

Wherever we return a SearchResult type in our schema, we might get a Human, a Droid, or a Starship. Note that members of a union type need to be concrete object types; you can’t create a union type out of interfaces or other unions.

In this case, if you query a field that returns the SearchResult union type, you need to use an inline fragment to be able to query any fields at all:

{
  search(text: "an") {
    __typename
    ... on Human {
      name
      height
    }
    ... on Droid {
      name
      primaryFunction
    }
    ... on Starship {
      name
      length
    }
  }
}

The __typename field resolves to a string which lets you differentiate different data types from each other on the client.