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Use cases for Union types have been discussed in the schema reference. Here, we discuss how to query union types. Let’s take this modified schema from the Substrate tutorial:
schema.graphql
type Account @entity {
  id: ID! #Account address
  events: [Event]
}

type WorkReport {
  id: ID! #event id
  addedFiles: [[String]]
  deletedFiles: [[String]]
  extrinsicId: String
  blockHash: String!
}

type JoinGroup {
  id: ID!
  owner: String!
  extrinsicId: String
  blockHash: String!
}

type StorageOrder {
  id: ID!
  fileCid: String!
  extrinsicId: String
  blockHash: String!
}

union Event = WorkReport | JoinGroup | StorageOrder
Here, an Event will have different fields depending on the underlying type. This query demonstrates how to request different fields for each of these types: 
query MyQuery {
  accounts {
    events {
      __typename
      ... on WorkReport {
        id
        blockHash
        extrinsicId
        deletedFiles
      }
      ... on JoinGroup {
        id
        blockHash
        extrinsicId
      }
      ... on StorageOrder {
        id
        blockHash
        extrinsicId
      }
    }
    id
  }
}
The special __typename field allows users to discern the returned object type without relying on comparing the sets of regular fields. For example, in the output of the query above JoinGroup and StorageOrder events can only be distinguished by looking at the __typename field. Here is a possible output to illustrate: 
{
  "data": {
    "accounts": [
      {
        "events": [
          {
            "__typename": "WorkReport",
            "id": "0000584321-000001-01cdb",
            "blockHash": "0x01cdb3cb6fa00f62fd20220104f1d740a53518b63517419da8a89325d065562b",
            "extrinsicId": "0000584321-000001-01cdb",
            "deletedFiles": []
          }
        ],
        "id": "cTKmzHG3RHa1yhujyZpPnNL17p8a48Av3JFwDjpttLcxeSo26"
      },
      {
        "events": [
          {
            "__typename": "JoinGroup",
            "id": "0000584598-000010-d06ec",
            "blockHash": "0xd06ec6716e96108e24987ef03d23c857ef3b467dd057d7a32c4e123fe5a8df36",
            "extrinsicId": "0000584598-000004-d06ec"
          }
        ],
        "id": "cTKqevWRdvbNNAQ3hLxhsNYhQ8pf5YGkYnnVjgjLNiVr4kd7a"
      },
      {
        "events": [
          {
            "__typename": "StorageOrder",
            "id": "0000584627-000013-1fa19",
            "blockHash": "0x1fa19ae98731afad853ffd491fcbc0c3dcda6b8b7f5a2d56ac6c4c1eb9e4f95e",
            "extrinsicId": "0000584627-000005-1fa19"
          }
        ],
        "id": "cTGYF8jvcpnRmgNopqT4nVs5rWHEviAAdRdfNrZE8NFz2Av7B"
      }
    ]
  }
}