Linq Join() method with left join and grouping

Question

I am trying to join two tables with grouping as per following sample. What i need to do is to apply Left join. tried several times but failed

 var _result = _parents
                    .Join(_childs, p => p.PID, c => c.PID, ((p, c) => new { p, c }) )
                    .GroupBy(x => new { x.p.Category })
                    .Select(y =>
                    {
                        var _projs = y.Where(z => (z.p.Completed == false || z.p.Completed is null)).Select(z => z.p.Action);
                        var _actBys = y.Where(z => (z.p.Completed == false || z.p.Completed is null)).Select(z => z.pj.ActionBy);
                        var _nas = y.Where(z => (z.p.Completed == false || z.p.Completed is null)).Select(z => z.c.Action);

                        return new
                        {

                             Item= y.Key.Category,
                            Projects = string.Join(" + ", _projs.Distinct()),
                            ActionBys = string.Join(" + ", _actBys.Distinct()),
                            NextActions = string.Join(" + ", _nas),
                        };
                    });

If it is not possible to apply Left join on this query, is it possible to get the same query with applying left join using GroupJoin?

Answer 1

I managed to get the required result but in two steps. First query left join Parents and Childs second step grouping in the way mentioned in the question.