SELECT clause, groupe FROM base GROUP BY clause
SELECT groupe from base GROUP BY groupe ORDER BY id
$mysqli->query("SELECT Groupe FROM base GROUP BY Groupe ORDER BY ID") or die($mysqli->error);
Expression #1 of ORDER BY clause is not in GROUP BY clause and contains nonaggregated column 'yoanpg.base.ID' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
SELECT ID,Groupe FROM base GROUP BY Groupe ORDER BY ID