What is the difference between a query and a mutation, and how do you test each?

Queries are reads — by convention free of side effects. Mutations are writes — they create, update or delete and then return a payload describing the result.

Testing a query: verify the requested fields come back correctly shaped, nesting resolves, nullability is honoured, and pagination/filtering arguments work.

Testing a mutation needs an extra step:

1. Run the mutation and assert on its return payload (status field, returned object, errors).
2. Run a follow-up query to confirm the change actually persisted.

The reason for step 2: a mutation resolver can return an optimistic success payload while the underlying write silently fails, or partially applies. Only a read-back proves the state changed. This is the GraphQL equivalent of the REST habit of GET-after-POST.

Also test mutation idempotency where it matters (e.g. a "create" that's accidentally run twice) and the error path (invalid input rejected with a useful code).