Why mutations modify data in GraphQL

Introduction

Mutations change data because they tell the system to add, update, or remove information. This helps keep data current and accurate.

When you want to add a new user to a website.

When you need to update a product's price in a store.

When you want to delete an old comment from a post.

When you want to change your profile information.

When you want to save a new order in an online shop.

Syntax

GraphQL

mutation {
  mutationName(input: {field1: value1, field2: value2}) {
    returnedField
  }
}

Mutations always start with the keyword mutation.

You provide input data inside parentheses to tell what to change.

Examples

This adds a new user named Alice who is 30 years old and returns her id and name.

GraphQL

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

This updates the price of the product with id 123 to 19.99 and returns the id and new price.

GraphQL

mutation {
  updateProduct(input: {id: "123", price: 19.99}) {
    id
    price
  }
}

This deletes the comment with id 456 and returns if it was successful and a message.

GraphQL

mutation {
  deleteComment(input: {id: "456"}) {
    success
    message
  }
}

Sample Program

This mutation creates a new post with a title and content, then returns the post's id, title, and content.

GraphQL

mutation {
  createPost(input: {title: "Hello World", content: "My first post"}) {
    id
    title
    content
  }
}

Important Notes

Mutations are different from queries because they change data, while queries only read data.

Always check the response to confirm your data changed as expected.

Summary

Mutations let you add, update, or delete data.

They use the mutation keyword and take input to know what to change.

Use mutations whenever you want to modify stored information.

Practice

1. What is the main purpose of a mutation in GraphQL?

easy

A. To modify data by adding, updating, or deleting it

B. To only fetch data without changes

C. To define the schema structure

D. To create a new query type

Solution

1. Step 1: Understand the role of mutations

   Mutations are designed to change data, unlike queries which only fetch data.

2. Step 2: Identify the correct purpose

   Since mutations add, update, or delete data, their main purpose is to modify data.

3. Final Answer:

   To modify data by adding, updating, or deleting it -> Option A

4. Quick Check:

   Mutation = Modify data [OK]

Hint: Mutations always change data, queries only read data [OK]

Common Mistakes:

• Confusing mutations with queries
• Thinking mutations only fetch data
• Believing mutations define schema only

2. Which of the following is the correct way to start a mutation operation in GraphQL?

easy

A. modify { updateUser(id: 1) { name } }

B. query { updateUser(id: 1) { name } }

C. update { updateUser(id: 1) { name } }

D. mutation { updateUser(id: 1) { name } }

Solution

1. Step 1: Identify the keyword for mutations

   GraphQL uses the keyword mutation to start mutation operations.

2. Step 2: Check the options

   Only mutation { updateUser(id: 1) { name } } uses the correct mutation keyword to start the operation.

3. Final Answer:

   mutation { updateUser(id: 1) { name } } -> Option D

4. Quick Check:

   Mutation keyword = mutation [OK]

Hint: Mutations always start with the 'mutation' keyword [OK]

Common Mistakes:

• Using 'query' instead of 'mutation'
• Using non-existent keywords like 'update' or 'modify'
• Omitting the mutation keyword

3. Given the mutation below, what will be the result if the user with id 2 exists?

mutation {
  updateUser(id: 2, input: {name: "Alice"}) {
    id
    name
  }
}

medium

A. Syntax error due to missing input type

B. { "data": { "updateUser": null } }

C. { "data": { "updateUser": { "id": 2, "name": "Alice" } } }

D. No change, returns old user data

Solution

1. Step 1: Understand mutation behavior

   The mutation updates user with id 2, changing the name to "Alice".

2. Step 2: Predict the returned data

   The mutation returns the updated user data with id and new name.

3. Final Answer:

   { "data": { "updateUser": { "id": 2, "name": "Alice" } } } -> Option C

4. Quick Check:

   Mutation updates and returns new data [OK]

Hint: Mutations return updated data if successful [OK]

Common Mistakes:

• Assuming mutation returns null on success
• Thinking mutation causes syntax error here
• Believing mutation returns old data without changes

4. Identify the error in this mutation:

mutation {
  addUser(name: "Bob")
  {
    id
    name
  }
}

medium

A. Mutation keyword is incorrect

B. Missing input argument object for addUser

C. Curly braces are misplaced

D. No error, mutation is correct

Solution

1. Step 1: Check mutation input format

   Mutations usually require input as an object, e.g., addUser(input: {name: "Bob"}).

2. Step 2: Identify missing input argument

   The mutation lacks the input argument object, causing an error.

3. Final Answer:

   Missing input argument object for addUser -> Option B

4. Quick Check:

   Input argument missing = error [OK]

Hint: Mutations need input objects for data changes [OK]

Common Mistakes:

• Forgetting to wrap inputs in an object
• Misplacing curly braces
• Confusing mutation keyword

5. You want to delete a user and then immediately fetch the updated user list in one mutation operation. Which approach correctly uses mutations to achieve this?

hard

A. Use a single mutation with two fields: deleteUser and users to delete and fetch updated list

B. Use a query to delete the user and a mutation to fetch the list

C. Use two separate mutations, one for deleting and one for fetching users

D. Use a mutation to delete the user and a query to fetch the updated list separately

Solution

1. Step 1: Understand mutation capabilities

   GraphQL mutations can include multiple fields to perform several operations in one request.

2. Step 2: Identify correct approach

   Using a single mutation with deleteUser and users fields allows deleting and fetching updated data atomically.

3. Final Answer:

   Use a single mutation with two fields: deleteUser and users to delete and fetch updated list -> Option A

4. Quick Check:

   Single mutation can do multiple related operations [OK]

Hint: Combine delete and fetch in one mutation for atomic update [OK]

Common Mistakes:

• Trying to delete with a query
• Splitting operations into separate requests unnecessarily
• Using mutation only for delete without fetching updated data