Mutation syntax in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this mutation?

Given this GraphQL mutation to add a new user:

mutation {
  addUser(name: "Alice", age: 30) {
    id
    name
    age
  }
}

Assuming the server assigns id 101 to the new user, what is the expected output?

GraphQL

mutation {
  addUser(name: "Alice", age: 30) {
    id
    name
    age
  }
}

A { "data": { "addUser": { "id": 101, "name": "Alice", "age": 30 } } }

B { "data": { "addUser": { "name": "Alice", "age": 30 } } }

C { "data": { "addUser": { "id": "101", "name": "Alice", "age": "30" } } }

D { "error": "Field 'id' not found" }

💡 Hint

Remember the server returns all requested fields with their correct types.

The mutation returns the new user object with all requested fields including the server-assigned id as an integer.

📝 Syntax

intermediate

Identify the syntax error in this mutation

Which option contains a syntax error in this GraphQL mutation?

mutation {
  updateUser(id: 5, name: "Bob") {
    id
    name
  }
}

GraphQL

mutation {
  updateUser(id: 5, name: "Bob") {
    id
    name
  }
}

A mutation { updateUser(id: 5, name: "Bob") { id name }

B mutation { updateUser(id: 5 name: "Bob") { id name } }

C mutation { updateUser(id: 5, name: "Bob") { id name } }

D mutation updateUser { id: 5, name: "Bob" }

💡 Hint

Check commas between arguments and braces.

Option B is missing a comma between arguments 'id: 5' and 'name: "Bob"', causing a syntax error.

❓ optimization

advanced

Optimize this mutation to reduce response size

You want to update a user's email but only need the updated email in the response. Which mutation is optimized for minimal response size?

A mutation { updateUser(id: 10, email: "[e-mail]") { id name email age } }

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

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

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

💡 Hint

Request only the fields you need to reduce data transfer.

Option D requests only the updated email field, minimizing the response size.

🔧 Debug

advanced

Why does this mutation cause an error?

Consider this mutation:

mutation {
  deleteUser(id: "abc") {
    id
  }
}

It causes an error. What is the most likely reason?

GraphQL

mutation {
  deleteUser(id: "abc") {
    id
  }
}

A The mutation is missing a closing brace.

B The deleteUser field requires a name argument.

C The id argument should be an integer, not a string.

D The id argument is missing.

💡 Hint

Check the expected type of the id argument.

The id argument expects an integer, but a string was provided, causing a type error.

🧠 Conceptual

expert

What happens if a mutation requests a non-existent field?

Given this mutation:

mutation {
  addUser(name: "Eve", age: 25) {
    id
    nickname
  }
}

Assuming 'nickname' is not a valid field on User, what will the server respond?

GraphQL

mutation {
  addUser(name: "Eve", age: 25) {
    id
    nickname
  }
}

A { "errors": [{ "message": "Cannot query field 'nickname' on type 'User'." }] }

B { "error": "Field 'nickname' is deprecated" }

C { "data": { "addUser": { "id": 102 } } }

D { "data": { "addUser": { "id": 102, "nickname": null } } }

💡 Hint

GraphQL validates requested fields against the schema before execution.

Requesting a non-existent field causes a validation error and the server returns an error message.