Web Forms :: Selecting Multiple Items In Listbox?

I have a dataTable that contains a specific column from a database. A listbox contains 100 list items in it. When page loads, I need those listbox items to be selected, whose value matches the value in the datacolumn of the datatable.

Following is the code i tried, but is not working. For some reason, it selects only the last item in the datacolumn.

if (datTableRelatedSchool.Rows.Count == 0)
{
}
else
{
foreach (DataRow row in datTableRelatedSchool.Rows)
{
foreach (DataColumn myCol in datTableRelatedSchool.Columns)
{
lstRelatedSchool.SelectedValue = row["RelatedSchoolPK"].ToString();
}
}
}