SQL: GroupBy with ordinal returns an error

[matriv]

SELECT gender AS g,
  SUM({fn CONVERT(emp_no, SQL_BIGINT)}) AS "sum"
FROM test_emp
GROUP BY 1
HAVING (NOT (SUM({fn CONVERT(emp_no, SQL_BIGINT)}) IS NULL));

throws:

java.sql.SQLDataException: line 4:10: Group ordinal 1 refers to an aggregate function Alias which is not compatible/allowed with GROUP BY

[elasticmachine]

Pinging @elastic/es-search