Why mutations modify data in GraphQL - Challenge Your Understanding

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of Mutations in GraphQL

Why do mutations exist in GraphQL?

A To modify or create data on the server

B To only fetch data without changes

C To define the schema structure

D To handle client-side caching

💡 Hint

Think about what changes data on the server.

Mutations are designed to change data on the server, such as creating, updating, or deleting records. Queries only fetch data without modifying it.

❓ query_result

intermediate

Mutation Result Output

Given this mutation:

mutation { addBook(title: "New Book", author: "Jane") { id title author } }

What is the expected output if the mutation succeeds?

A {"error":"Cannot fetch data"}

B {"data":{"addBook":null}}

C {"data":{"addBook":{"id":"123","title":"New Book","author":"Jane"}}}

D {"data":{"books":[{"title":"New Book"}]}}

💡 Hint

Mutations return the changed data as specified in the selection set.

When a mutation succeeds, it returns the fields requested inside the mutation's selection set. Here, it returns the new book's id, title, and author.

📝 Syntax

advanced

Identify the Syntax Error in Mutation

Which option contains a syntax error in this mutation?

mutation { updateUser(id: "1", name: "Alice") { id name } }

A mutation { updateUser(id: "1", name: "Alice") { id, name } }

B mutation { updateUser(id: 1, name: "Alice") { id name } }

C mutation { updateUser(id: "1", name: "Alice") { id name } }

D mutation { updateUser(id: "1" name: "Alice") { id name } }

💡 Hint

Look for missing punctuation between arguments.

Option D is missing a comma between arguments, causing a syntax error. All other options are syntactically valid.

❓ optimization

advanced

Optimizing Mutation Response Size

You want to reduce the data returned by a mutation to improve performance. Which mutation selection set is best?

A mutation { deletePost(id: "5") { id content } }

B mutation { deletePost(id: "5") { id } }

C mutation { deletePost(id: "5") { id title } }

D mutation { deletePost(id: "5") { id title content author { id name } } }

💡 Hint

Only request the fields you need.

Requesting only the id field returns the smallest response, reducing data transfer and improving performance.

🔧 Debug

expert

Why Does This Mutation Not Modify Data?

A mutation is sent:

mutation { updateEmail(id: "10", email: "[e-mail]") { id email } }

But the email does not change in the database. What is the most likely reason?

A The mutation resolver does not update the database correctly

B The mutation name is invalid and causes a syntax error

C The query is missing a selection set

D The client did not send the mutation request

💡 Hint

Check the server-side code that handles the mutation.

If the mutation runs without error but data does not change, the resolver likely does not perform the update operation properly.