How can I filter table and select rows which have parentid in id column

Question

My table has tow columns id, parentid and I want to filter data to select rows which have parentid ( as there can be rows without parentid row).

id parentid
1  null
2   1
3   1
4   2
5   10

I want all the rows except 5, 10 as 10 is not there as a parent (no children unless parent is there).

I want rows with parentid as I am using the data for treeview. I have tried couple of ways but not getting what I want.

var mycol = from t in ds.Tables[0].AsEnumerable()
    where t.Field<int>("Id") == t.Fields<int>("ParentId");
    select t;

or

DataRow[] drs = ds.Tables[0].Select("ParentId in Id");

Answer 1

To get rows with ParentId not null, use

var rows = from r in ds.Tables[0].Rows.AsEnumerable()
    where !r.IsNull("ParentId")
    select r;

or

var rows = ds.Tables[0].Rows.AsEnumerable().Where(r => !r.IsNull("ParentId"));

Answer 2

It's unclear what you want.

If you want rows where ParentId is not null, use one of the other answers.

If you want rows where Id is used as a ParentId somewhere in the dataset, use something like this:

var table = ds.Tables[0];

var parentIds = new HashSet<int>(
    from row in table.AsEnumerable()
    let parentId = row.Field<int?>("ParentId")
    where parentId != null
    select parentId.Value);

var rowsWhereIdIsAParentId =
    from row in table.AsEnumerable()
    let id = row.Field<int>("Id")
    where parentIds.Contains(id)
    select row;