SQL конвертируется в LINQ Online
Select pp.PartId, Count(rp.Id) from ProjectParts pp
inner join Parts p on pp.PartId = p.Id
left join RFQPlans rp on rp.PartId = pp.PartId
Where pp.ProjectId = 7 And pp.IsDeleted = 0 And rp.RFQId is null
GROUP BY pp.PartId
Ram Parshad