What is the difference between Mutation and Query

Short

Conventionally:

• Query — for querying data (SELECT operations)
• Mutation — for creating new and updating/deleting existing data (INSERT, UPDATE, DELETE)

Detailed

Technically any GraphQL query could be implemented to cause a data write. But there is a convention that any operations that cause writes should be sent explicitly via a mutation.

Besides the difference in the semantic, there is one important technical difference:

Query fields can be executed in parallel by the GraphQL engine while Mutation top-level fields MUST execute serially according to the spec:

If the operation is a mutation, the result of the operation is the result of executing the mutation’s top level selection set on the mutation root object type. This selection set should be executed serially.

It is expected that the top level fields in a mutation operation perform side‐effects on the underlying data system. Serial execution of the provided mutations ensures against race conditions during these side‐effects.

Source: https://graphql.github.io/graphql-spec/draft/#sec-Mutation

In simple words the query is SELECT statement and mutation is INSERT Operation.

Query in graphql is used to fetch data while mutation is used for INSERT/UPDATE/DELETE operation.

query = SELECT

mutation = INSERT, UPDATE, DELETE