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To order through a left join in GraphQL Apollo Federation, you can follow these steps:

1. Define a custom scalar for the order value (e.g. OrderValue) in the schema.
2. Define a sortDirection enum with ASC and DESC.
3. Set up your federated services and relationships in the federation schema.
4. Write a query that includes the fields to order and the join relationship.
5. Define a custom field resolver for the parent service that retrieves the child service data and performs the sorting.
6. Define a custom field resolver for the child service that retrieves the parent service data and performs the sorting.
7. Add arguments to the query that allow the sorting to be customized (e.g. orderBy: OrderValue, sortDirection: sortDirection).
8. Handle error conditions, such as if the child service data is missing or if the sorting parameter is invalid.

By following these steps, you can enable ordering through a left join in your GraphQL Apollo Federation API.