Union types in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query using union types?

Given the schema with a union SearchResult = Photo | Person, and the query below, what is the expected output?

GraphQL

query {
  search(text: "john") {
    ... on Photo {
      id
      url
    }
    ... on Person {
      id
      name
    }
  }
}

A [{"id": "p1", "name": "John Doe"}, {"id": "u1", "url": "http://example.com/photo1.jpg"}]

B ]}"gpj.1otohp/moc.elpmaxe//:ptth" :"lru" ,"1u" :"di"{ ,}"eoD nhoJ" :"eman" ,"1p" :"di"{[

C [{"id": "p1", "url": "http://example.com/photo1.jpg", "name": "John Doe"}]

D SyntaxError: Cannot query fields on union type without inline fragments

💡 Hint

Remember that when querying union types, you must use inline fragments to specify fields for each type.

The query uses inline fragments to select fields from each type in the union. The result is a list of objects matching either Photo or Person with their respective fields.

🧠 Conceptual

intermediate

Which statement about GraphQL union types is true?

Choose the correct statement about union types in GraphQL.

A Union types can include scalar types like String or Int.

B Union types are used to define fields that return lists of a single object type.

C Union types require all member types to share the same fields.

D Union types allow a field to return one of several object types but cannot include interfaces.

💡 Hint

Think about what types can be included in a union and how they differ from interfaces.

Union types can only include object types, not scalar types or interfaces. They allow a field to return one of several object types without requiring shared fields.

📝 Syntax

advanced

Identify the syntax error in this GraphQL union type definition

Which option shows the correct syntax for defining a union type SearchResult that includes Photo and Person?

GraphQL

union SearchResult = Photo | Person

A union SearchResult = Photo | Person

B union SearchResult: Photo | Person

C union SearchResult { Photo | Person }

D type SearchResult = Photo | Person

💡 Hint

Recall the exact syntax for union type declarations in GraphQL SDL.

The correct syntax uses the keyword union, the union name, an equals sign, and the member types separated by pipes.

❓ optimization

advanced

How to optimize a query using union types to reduce data transfer?

You have a union type SearchResult = Photo | Person. You want to fetch only the id and name for Person and only the id and url for Photo. Which query is the most efficient?

A query { search(text: "abc") { ... on Person { id name url } ... on Photo { id url name } } }

B query { search(text: "abc") { id name url } }

C query { search(text: "abc") { ... on Person { id name } ... on Photo { id url } } }

D query { search(text: "abc") { id } }

💡 Hint

Think about how inline fragments help fetch only needed fields per type.

Using inline fragments to specify fields per type avoids fetching unnecessary fields, reducing data transfer.

🔧 Debug

expert

Why does this GraphQL query with union types fail?

Given the union type SearchResult = Photo | Person, why does this query fail?

query {
  search(text: "xyz") {
    id
    name
  }
}

A Because the search field does not accept arguments.

B Because fields must be queried inside inline fragments for union types.

C Because the query is missing the __typename field.

D Because the union type must be replaced with an interface in the query.

💡 Hint

Recall how to query fields on union types in GraphQL.

When querying union types, you must use inline fragments to specify fields for each possible type. Querying fields directly on the union causes an error.