Aggregate query has too many rows for direct assignment but less than 200 rows

A SOQL query throws the exception "Aggregate query has too many rows for direct assignment" in two different scenarios:

Scenario 1: Subselect Returns More Than 200 Rows

When a subselect in a SOQL aggregate query returns more than 200 rows, Salesforce raises this exception when you attempt to assign the results directly to a variable. To resolve this, iterate over the results using a FOR loop instead of direct assignment.

Scenario 2: Subselect Returns Fewer Than 200 Rows — Long Text or Rich Text Fields

If all subselects are returning fewer than 200 rows but the exception still occurs, the issue may be that the SOQL query is selecting Long Text Area or Rich Text Area fields.
These field types can store large amounts of data. Salesforce applies additional memory constraints to queries that reference many large object (LOB) fields to prevent excessive memory consumption. This constraint can reduce the effective row limit of the subselect below 200, causing the exception to be raised even when the actual row count appears low.