Через query. Примерно так:
PHP код:
Query query;
QueryBuildDatasource qbds;
QueryRun queryRun;
Table table;
query = new Query();
qbds = query.addDatasource(tableNum( Table ));
qbds.addSelectionField(fieldId, SelectionField::Sum);
queryRun = new QueryRun(query);
queryRun.next();
table = queryRun.getNo(1);
info(strFmt("%1", table.(fieldId) ));