Question

For simplicity, a “Section” object contains the following properties:

SectionId
ParentSectionId
Name

I currently have the following LINQ code to obtain child sections of a given section:

List<Section> sections = SectionCache.GetAllSections();

sections.AsQueryable().Where(s => s.ParentSectionId == 10);

This gives me all children of the section with a SectionId of 10 (again, for simplicity), but I need to develop this further to only include sections that themselves have children. In SQL, I might do something like this:

SELECT      Section.SectionId,
            Section.ParentSectionId,
            Section.Name
FROM        Section
INNER JOIN  Section children ON children.ParentSectionId = Section.SectionId
WHERE       Section.ParentSectionId = 10
GROUP BY    Section.SectionId,
            Section.ParentSectionId,
            Section.Name
HAVING      COUNT(children.SectionId) > 0

How can I achieve this in LINQ/what is the best way to achieve this with LINQ?

Thanks

Answer 1

Here’s a couple ways of doing it (these are pretty much identical)

IEnumerable<Section> query =
  sections.Where(s =>
    sections.Any(c => s.SectionId = c.ParentSectionId))
  );

IEnumerable<Section> query =
  from s in sections
  where (
   from c in sections
   where c.ParentSectionId == s.SectionId)
   select c).Any()
  select s;

Or more optimally:

ILookup<int, Section> childLookup =
  sections.ToLookup(c => c.ParentSectionId);
IEnumerable<Section> query =
  sections.Where(s => childLookup[s.SectionId].Any());

Then there’s the groupjoin technique, which should be pretty efficient too:

IEnumerable<Section> query =
  from s in sections
  join c in sections
    on s.SectionId equals c.ParentSectionId
    into children
  where children.Any()
  select s;