No hay que estar adivinando, a menos que se use el sql de fox, el cual es el menos estandar de todos, la cosa es así:
Cita: GROUP BY Clause
Specifies the groups into which output rows are to be placed and, if aggregate functions are included in the SELECT clause <select list>, calculates a summary value for each group. When GROUP BY is specified, either each column in any non-aggregate expression in the select list should be included in the GROUP BY list, or the GROUP BY expression must match exactly the select list expression.