Question

I need to write it with a single query.
Depending on the parameter regionid must hold one of the conditions if the parameter is greater than zero regionid if this parameter is involved in the request if there is not involved.

 var myRegionId = 0;
                if (!string.IsNullOrEmpty(regionId))
                {
                    myRegionId = int.Parse(regionId);
                }

                IOrderedQueryable price;
                if (myRegionId>0)
                {
                     price = (from p in _db.PRICEs
                                  join good in _db.GOODs on p.good_id equals good.id
                                  join gname in _db.spr_goods_names on good.goods_name_id equals gname.id
                                  where ******p.region_id == myRegionId &&** gname.name.ToLower().Contains(filterText.ToLower())****
                                  group p by new{ p.good_id} into g
                                  select new
                                  {
                                      GoodId = g.Key.good_id,
                                      Promotion = g.Count(x => x.promotion != ""),
                                      MinPrice = g.Min(x => x.good_price),
                                      DistributorCount = g.Count(x => x.distributor_id != null)
                                  }
                            ).OrderByDescending(x => x.DistributorCount).Take(100).OrderBy(x => x.MinPrice);
                }else
                {
                    price = (from p in _db.PRICEs
                             join good in _db.GOODs on p.good_id equals good.id
                             join gname in _db.spr_goods_names on good.goods_name_id equals gname.id
                             **where gname.name.ToLower().Contains(filterText.ToLower())**
                             group p by new { p.good_id } into g
                             select new
                             {
                                 GoodId = g.Key.good_id,
                                 Promotion = g.Count(x => x.promotion != ""),
                                 MinPrice = g.Min(x => x.good_price),
                                 DistributorCount = g.Count(x => x.distributor_id != null)
                             }
                           ).OrderByDescending(x => x.DistributorCount).Take(100).OrderBy(x => x.MinPrice);
                }

Answer 1

How about this?

 price = (from p in _db.PRICEs
          join good in _db.GOODs on p.good_id equals good.id
          join gname in _db.spr_goods_names on good.goods_name_id equals gname.id
          where (myRegionId <= 0 || p.region_id == myRegionId) && gname.name.ToLower().Contains(filterText.ToLower())
          group p by new{ p.good_id} into g
          select new
          {
              GoodId = g.Key.good_id,
              Promotion = g.Count(x => x.promotion != ""),
              MinPrice = g.Min(x => x.good_price),
              DistributorCount = g.Count(x => x.distributor_id != null)
          }).OrderByDescending(x => x.DistributorCount).Take(100).OrderBy(x => x.MinPrice);

if myRegionId <= 0, then it evaluates to true and p.region_id == myRegionId is not evaluated and thus ignored.