The DefraDB Query Language (DQL) allows you to store and query data in a DefraDB node. Based on [GraphQL](https://graphql.org), at a high level it has two type of operations: _query_ and _mutation_ blocks.

- **Query blocks** – The *read* side of DQL. Retrieve data without the ability to alter it.
- **Mutation blocks** – The *write* side of DQL. Mutations rely on the query system to pinpoint the data to update or delete.

As GraphQL queries and mutations must always return some fields, all blocks return a JSON object:

```json title="Return object structure"
{
  "data": {
    "<type>": [
      <documentObject>
    ]
  }
}
```

- `<type>` &ndash; Object type, such as a [collection](/schema/collections.md) name.
- `<documentObject>` &ndash; Return object for each selected document.

In case of error, the `data` object is `null` and the `error` object contains a list of error objects:

```json title="Error object structure"
{
  "errors": [
    {
      "message": "Field \"Book\" of type \"[Book]\" must have a sub selection."
    }
  ],
  "data": null
}
```

An extra field `extensions` might be present in responses if extra information (such as warnings) is surfaced by the server.

```json title='Result containing "extensions" field'
"extensions": {
  "warnings": [
    {
      "code": "VECTOR_INDEX_UNUSED",
      "message": "similarity query on field 'about_v' did not use the vector index and read the whole collection",
      "detail": {
        "field": "about_v",
        "reason": "notOrderedBySimilarityDesc"
      }
    }
  ]
}
```