What is the method to query a field that is not empty?

In Elasticsearch, you can use the exists query to check if a field exists in order to determine if the field is empty. The specific query method is as follows:

GET /index/_search
{
  "query": {
    "exists": {
      "field": "fieldName"
    }
  }
}

The index is the name of the index to be queried, and fieldName is the name of the field to be checked. If the field exists and is not empty, it will return the matching document.