Field selection in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this GraphQL query?

Given the following GraphQL schema snippet:

type Book {
  title: String
  author: Author
}

type Author {
  name: String
  age: Int
}

And the query:

{
  book {
    title
    author {
      name
    }
  }
}

What fields will be included in the response?

A {"data": {"book": {"title": "1984", "author": {"name": "George Orwell"}}}}

B {"data": {"book": {"title": "1984", "author": {"name": "George Orwell", "age": 46}}}}

C {"data": {"book": {"title": "1984"}}}

D {"data": {"book": {"author": {"name": "George Orwell"}}}}

💡 Hint

Look carefully at which fields are requested inside author.

The query asks for title and inside author only name. So the response includes those fields only.

📝 Syntax

intermediate

Which GraphQL query has a syntax error?

Identify the query that will cause a syntax error when executed.

A { book { title author name } }

B { book { title author { name age } } }

C } } } eman { rohtua eltit { koob {

D { book { title author { name } } }

💡 Hint

Remember that nested fields must be enclosed in braces.

Option A tries to select author and name at the same level without nesting name inside author. This is invalid syntax.

❓ optimization

advanced

Which query is the most efficient to fetch only book titles?

You want to fetch only the titles of all books. Which query is best to minimize data transfer?

A { books { title author { name } } }

B { books { title author { name age } } }

C { books { title author } }

D { books { title } }

💡 Hint

Request only the fields you need.

Option D requests only title for each book, minimizing data sent. Other options request extra fields unnecessarily.

🧠 Conceptual

advanced

What happens if you omit a field in a GraphQL query?

Consider a GraphQL type User with fields id, name, and email. If you query only id and name, what will the server return for email?

A The server returns the email field with its value.

B The server returns null for the email field.

C The server omits the email field entirely from the response.

D The server returns an error because email was not requested.

💡 Hint

GraphQL only returns fields explicitly requested.

If a field is not requested in the query, the server does not include it in the response at all.

🔧 Debug

expert

Why does this GraphQL query return an error?

Given the schema:

type Query {
  user(id: ID!): User
}

type User {
  id: ID
  name: String
  friends: [User]
}

And the query:

{
  user(id: "1") {
    id
    name
    friends {
      id
      name
      friends {
        id
      }
    }
  }
}

The server returns an error about exceeding query depth. Why?

A The query is missing required arguments for friends field.

B The query requests nested friends fields recursively, causing too deep nesting.

C The query uses invalid field names for User type.

D The query syntax is incorrect due to missing braces.

💡 Hint

Think about how deep the query goes into nested fields.

The query requests friends of friends of friends, which can cause very deep recursion. Many GraphQL servers limit query depth to prevent abuse.

Practice

1. What does field selection in GraphQL allow you to do?

easy

A. Encrypt data before sending it

B. Automatically update the database schema

C. Choose only the data fields you want to receive

D. Create new database tables

Solution

1. Step 1: Understand the purpose of field selection

   Field selection lets you specify exactly which fields you want in your query response.

2. Step 2: Compare options with the concept

   Only Choose only the data fields you want to receive describes choosing specific data fields, which matches field selection.

3. Final Answer:

   Choose only the data fields you want to receive -> Option C

4. Quick Check:

   Field selection = Choose specific fields [OK]

Hint: Field selection means picking only needed data fields [OK]

Common Mistakes:

• Confusing field selection with schema changes
• Thinking it encrypts data
• Assuming it creates database tables

2. Which of the following is the correct GraphQL syntax to select the fields id and name from a user?

easy

A. { user: id, name }

B. { user(id, name) }

C. query user { id, name }

D. { user { id, name } }

Solution

1. Step 1: Recall GraphQL field selection syntax

   Fields are selected inside curly braces after the type name, separated by commas or spaces.

2. Step 2: Check each option's syntax

   { user { id, name } } correctly uses braces and field names inside user. Others have syntax errors or wrong structure.

3. Final Answer:

   { user { id, name } } -> Option D

4. Quick Check:

   Correct field selection syntax = { user { id, name } } [OK]

Hint: Use braces to nest fields inside the type name [OK]

Common Mistakes:

• Using colon instead of braces for fields
• Omitting braces around fields
• Trying to pass fields as function arguments

3. Given this GraphQL query:

{ book { title author { name } } }

What fields will be returned in the response?

medium

A. { "book": { "title": "...", "author": { "name": "..." } } }

B. { "book": { "title": "...", "author": "..." } }

C. { "book": { "author": { "name": "..." } } }

D. { "book": { "title": "..." } }

Solution

1. Step 1: Analyze the query structure

   The query requests title and nested author with its name field.

2. Step 2: Determine the response shape

   The response will include title and an author object containing name.

3. Final Answer:

   { "book": { "title": "...", "author": { "name": "..." } } } -> Option A

4. Quick Check:

   Nested fields return nested objects [OK]

Hint: Nested fields return nested objects in response [OK]

Common Mistakes:

• Expecting author as a string instead of object
• Missing requested fields in response
• Ignoring nested field structure

4. Identify the error in this GraphQL query:

{ user { id name, } }

medium

A. Trailing comma after last field causes syntax error

B. Missing parentheses after user

C. Fields must be quoted strings

D. Curly braces are not allowed inside queries

Solution

1. Step 1: Check field list syntax

   GraphQL does not allow a comma after the last field in a selection set.

2. Step 2: Verify other syntax rules

   No parentheses needed after user, fields are unquoted, and braces are required.

3. Final Answer:

   Trailing comma after last field causes syntax error -> Option A

4. Quick Check:

   No trailing commas in field lists [OK]

Hint: No trailing commas after last field in selection [OK]

Common Mistakes:

• Adding commas after last field
• Adding unnecessary parentheses
• Quoting field names

5. You want to fetch a list of products with only their id and price, but each product also has a nested manufacturer with name and location. How do you write a GraphQL query to select only id, price, and the manufacturer's name (not location)?

hard

A. { products { id price manufacturer { name location } } }

B. { products { id price manufacturer { name } } }

C. { products { id price manufacturer name } }

D. { products { id price, manufacturer: { name } } }

Solution

1. Step 1: Identify required fields

   We need id, price, and nested manufacturer with only name.

2. Step 2: Write correct nested field selection

   { products { id price manufacturer { name } } } correctly nests name inside manufacturer and excludes location.

3. Step 3: Check other options for errors

   { products { id price manufacturer { name location } } } includes unwanted location. { products { id price manufacturer name } } incorrectly places name outside manufacturer. { products { id price, manufacturer: { name } } } has invalid colon syntax.

4. Final Answer:

   { products { id price manufacturer { name } } } -> Option B

5. Quick Check:

   Select only needed nested fields [OK]

Hint: Nest only needed fields inside parent fields [OK]

Common Mistakes:

• Selecting unwanted nested fields
• Placing nested fields outside their parent
• Using colon instead of braces for nesting