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We’re overhauling Dgraph’s docs to make them clearer and more approachable. If you notice any issues during this transition or have suggestions, please let us know.
Dgraph Query Language (DQL) lets you build custom resolvers logic that goes beyond what’s possible with the GraphQL CRUD API. To define a DQL custom query, use the notation: 
 @custom(dql: """
  ...
  """)
Since v21.03, you can also subscribe to custom DQL queries.
For example, lets say you had following schema: 
type Tweets {
  id: ID!
  text: String! @search(by: [fulltext])
  author: User
  timestamp: DateTime! @search
}
type User {
  screen_name: String! @id
  followers: Int @search
  tweets: [Tweets] @hasInverse(field: author)
}
and you wanted to query tweets containing some particular text sorted by the number of followers their author has. Then, this isn’t possible with the automatically generated CRUD API. Similarly, let’s say you have a table sort of UI component in your app which displays only a user’s name and the number of tweets done by that user. Doing this with the auto-generated CRUD API would require you to fetch unnecessary data at client side, and then employ client side logic to find the count. Instead, all this could simply be achieved by specifying a DQL query for such custom use-cases. So, you would need to modify your schema like this: 
type Tweets {
  id: ID!
  text: String! @search(by: [fulltext])
  author: User
  timestamp: DateTime! @search
}
type User {
  screen_name: String! @id
  followers: Int @search
  tweets: [Tweets] @hasInverse(field: author)
}
type UserTweetCount @remote {
  screen_name: String
  tweetCount: Int
}

type Query {
  queryTweetsSortedByAuthorFollowers(search: String!): [Tweets]
    @custom(
      dql: """
      query q($search: string) {
        var(func: type(Tweets)) @filter(anyoftext(Tweets.text, $search)) {
          Tweets.author {
            followers as User.followers
          }
          authorFollowerCount as sum(val(followers))
        }
        queryTweetsSortedByAuthorFollowers(func: uid(authorFollowerCount), orderdesc: val(authorFollowerCount)) {
          id: uid
          text: Tweets.text
          author: Tweets.author {
              screen_name: User.screen_name
              followers: User.followers
          }
          timestamp: Tweets.timestamp
        }
      }
      """
    )

  queryUserTweetCounts: [UserTweetCount]
    @custom(
      dql: """
      query {
        queryUserTweetCounts(func: type(User)) {
          screen_name: User.screen_name
          tweetCount: count(User.tweets)
        }
      }
      """
    )
}
Now, if you run following query, it would fetch you the tweets containing “GraphQL” in their text, sorted by the number of followers their author has: 
query {
  queryTweetsSortedByAuthorFollowers(search: "GraphQL") {
    text
  }
}
There are following points to note while specifying the DQL query for such custom resolvers:
  • The name of the DQL query that you want to map to the GraphQL response, should be same as the name of the GraphQL query.
  • You must use proper aliases inside DQL queries to map them to the GraphQL response.
  • If you are using variables in DQL queries, their names should be same as the name of the arguments for the GraphQL query.
  • For variables, only scalar GraphQL arguments like Boolean, Int, Float, etc are allowed. Lists and Object types aren’t allowed to be used as variables with DQL queries.
  • You would be able to query only those many levels with GraphQL which you have mapped with the DQL query. For instance, in the first custom query in this example, we haven’t mapped an author’s tweets to GraphQL alias, so, we won’t be able to fetch author’s tweets using that query.
  • If the custom GraphQL query returns an interface, and you want to use __typename in GraphQL query, then you should add dgraph.type as a field in DQL query without any alias. This isn’t required for types, only for interfaces.
  • to subscribe to a custom DQL query, use the @withSubscription directive. See the Subscriptions article for more information.