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Select A.Id,Count(B.Id) from A Left Join B on B.Aid==A.id and B.Jid==3
Group By A.Id
var query = from a in A
join b in B
on new { a.id, jid = 3 } equals new { id = b.Aid, jid = b.Jid } into AB
from ab in AB.DefaultIfEmpty()
group ab by ab.id into g
select new { key = g.Key, count = g.Count(c=>c.Jid) };