How to use $expand query in OData? What is $expand in OData?

 In OData, the $expand query option is used to retrieve related entities along with the requested entities in a single round-trip to the OData service.

The basic syntax for using $expand is as follows:

bash
http://serviceRootURL/EntitySet?$expand=NavigationProperty1,NavigationProperty2...

where serviceRootURL is the base URL of the OData service, EntitySet is the name of the entity set you want to query, and NavigationProperty1, NavigationProperty2, etc. are the names of the navigation properties that represent the related entities you want to retrieve.

For example, let's say you have an OData service that exposes a collection of Orders, each of which has an associated Customer entity through a navigation property called Customer. If you want to retrieve all the orders in the collection along with the associated customer entities, you can use the following query:

bash
http://serviceRootURL/Orders?$expand=Customer

This will return a response that includes all the orders in the collection, with the associated customer entities embedded within each order entity.

Note that you can also use $expand to retrieve related entities at multiple levels of nesting by chaining navigation properties together. For example, if you have an Order entity that has an associated Customer entity, and the Customer entity in turn has associated Address and CreditCard entities, you can use the following query to retrieve all the orders along with their associated customer, address, and credit card entities:

bash
http://serviceRootURL/Orders?$expand=Customer($expand=Address,CreditCard)

This will return a response that includes all the orders in the collection, with the associated customer entities embedded within each order entity, and the associated address and credit card entities embedded within each customer entity.