SQL查出一个关于ids的list 去重

List parentIds = list.stream().map(PurchaseEntity::getParentId)
.collect(Collectors.toList());
parentIds.removeAll(Collections.singleton(null));
//去重parentIds
for (int i = 0; i < parentIds.size() - 1; i++) {
for (int j = parentIds.size() - 1; j > i; j–) {
if(parentIds.get(j).toString().equals(parentIds.get(i).toString())){
parentIds.remove(j);
}
}
}