var q = from c in customer
join o in order on o.fk_custId equals c.custId
join ol in orderLines on ol.fk_orderId equals o.orderId
join fi in foodItem on fi.foodId equals ol.fk_foodId
\\ where
group new { c, o, ol, fi } into g
select new
{
orderId= o.orderId,
date = o.date,
Total = g.Sum(x => x.fi.price* x.ol.quantity),
TotalPrice= g.Sum(x => x.ol.extraPrice + x.m.Total )
};
Table Joins are not specific. Please modify, as i was giving example. Please try this query.