Can you explain the difference between queries and mutations in GraphQL, and when you would use each?

In depth

GraphQL distinguishes between queries and mutations to provide clarity in operations. Queries are used to retrieve data, offering a way to specify exactly what data fields are needed, which can reduce over-fetching. Mutations, on the other hand, not only allow modifications to the data but also return a payload, typically the updated state of the data. This distinction supports a clear contract between the client and server, where the client can understand what data will change and how that change will be represented. Additionally, mutations can have side effects, such as triggering an update in a database, which queries do not perform.

Example scenario

In a social media application, a user might perform a query to retrieve their profile information and the latest posts. This could look like a request for fields like the username and post content. Conversely, when a user wants to add a new post, they would use a mutation. The mutation would send the new post data to the server, and in response, it might provide the updated list of posts, ensuring the client has the most recent data.

Common mistakes

A common mistake is using mutations when a query would suffice, which can lead to unnecessary updates and complications. For instance, a developer might try to fetch data using a mutation instead of designing a clear query structure. Another mistake is neglecting to handle the response from a mutation correctly; failing to do so can lead to the application displaying stale data since it does not refresh after a mutation is performed.

Follow-up questions

• Can you give an example of a mutation you might use in a web application?
• How do you handle errors in mutations?
• What tools can you use to document your GraphQL schema?
• Can you explain the importance of input types in mutations?

Related concepts: GraphQL schema, input types, error handling, resolvers