Final answer:
The _id field in the group by field of a MongoDB aggregation operation is mandatory. It is used as a grouping key and cannot be omitted if grouping is intended. To aggregate without specific grouping, _id: null may be used.
Step-by-step explanation:
The statement is False. In the context of database operations, specifically when using aggregation operations such as the $group stage in MongoDB's aggregation pipeline, the _id field in the group by field is indeed mandatory. This _id field serves as the grouping key and is required to specify the fields by which documents are grouped. Each unique combination of values in the specified _id field results in a distinct group. If you are looking to aggregate documents without grouping them, you would typically use { _id: null } which aggregates all documents into a single group.