FetchXML vs OData vs QueryExpression: which one when

Dataverse gives you several ways to ask for data. They all return the same rows, but each fits a different place in your solution.

The same query, three ways

Goal: the 10 newest active contacts, with name and email.

<fetch top="10">
  <entity name="contact">
    <attribute name="fullname" />
    <attribute name="emailaddress1" />
    <filter>
      <condition attribute="statecode" operator="eq" value="0" />
    </filter>
    <order attribute="createdon" descending="true" />
  </entity>
</fetch>

GET [org]/api/data/v9.2/contacts?$select=fullname,emailaddress1&$filter=statecode eq 0&$orderby=createdon desc&$top=10

var query = new QueryExpression("contact")
{
    ColumnSet = new ColumnSet("fullname", "emailaddress1"),
    TopCount = 10
};
query.Criteria.AddCondition("statecode", ConditionOperator.Equal, 0);
query.AddOrder("createdon", OrderType.Descending);

var contacts = service.RetrieveMultiple(query).Entities;

When to use each

Practical tips

• Copy, don't write. Build FetchXML in Advanced Find or with a tool like FetchXML Builder, then paste it. Use the formatter on the Dev tools page to tidy it up.
• Aggregates need FetchXML. Sums, counts and group-by are simplest with aggregate="true".
• Paging matters. Queries return 5,000 rows per page by default. Use paging cookies (FetchXML) or @odata.nextLink (Web API) when you need everything.
• In flows, List rows accepts both a FetchXML query and OData-style filter. Use FetchXML when you need joins.