code: public QueryBuilder<T, ID> groupBy(String columnName) {
  		FieldType fieldType = verifyColumnName(columnName);
  		if (fieldType.isForeignCollection()) {
  			throw new IllegalArgumentException("Can't groupBy foreign collection field: " + columnName);
  		}
  		addGroupBy(ColumnNameOrRawSql.withColumnName(columnName));
  		return this;
  	}
docs: Add "GROUP BY" clause to the SQL query statement. This can be called multiple times to add additional "GROUP BY"
  clauses.
  
  <p>
  NOTE: Use of this means that the resulting objects may not have a valid ID column value so cannot be deleted or
  updated.
  </p>