ASP - invalid in the select list because it is not contained in either an aggregate function or the GROUP

como agrego a la consulta el campo RMT_CAIN, me genera el sigiente error:

Column 'cont_tras.rmt_cain' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.



La consulta es la siguiente:

"SELECT RMT_CAIN, ref_prod, sum(cant_ing) as cant_tras FROM cont_tras where fec_est >='"&fec_ini&"' and fec_est <='"&fec_fin& "' group by ref_prod"

Gracias.

Cali - Colombia

"SELECT RMT_CAIN, ref_prod, sum(cant_ing) as cant_tras FROM cont_tras where fec_est >='"&fec_ini&"' and fec_est <='"&fec_fin& "' group by ref_prod, RMT_CAIN"

agregue el campo en el group by