LINQ with Group, Join and Where Easy in SQL not in LINQ? - performance

Question

I have trouble understand how to translate SQL into LINQ. I would like to do the following but can't figure out how to get the Group By to work
var query = from s in Supplier
join o in Offers on s.Supp_ID equals o.Supp_ID
join p in Product on o.Prod_ID equals p.Prod_ID
where s.City == "Chicago"
group s by s.City into Results
select new { Name = Results.Name };
I just need to do something simple like display the product name of this simple query, how does the group by work with joins and a where?

Answer 1

You haven't provided classes so I assumed that they are like below:
public class Supplier
{
public int SupplierID { get; set; }
public string SuppierName { get; set; }
public string City { get; set; }
}
public class Product
{
public int ProductID { get; set; }
public string ProductName { get; set; }
}
public class Offer
{
public int SupplierID { get; set; }
public int ProductID { get; set; }
}
Then I added data for testing:
List<Supplier> supplierList = new List<Supplier>()
{
new Supplier() { SupplierID = 1, SuppierName = "FirstCompany", City = "Chicago"},
new Supplier() { SupplierID = 2, SuppierName = "SecondCompany", City = "Chicago"},
new Supplier() { SupplierID = 3, SuppierName = "ThirdCompany", City = "Chicago"},
};
List<Product> productList = new List<Product>()
{
new Product() { ProductID = 1, ProductName = "FirstProduct" },
new Product() { ProductID = 2, ProductName = "SecondProduct" },
new Product() { ProductID = 3, ProductName = "ThirdProduct" }
};
List<Offer> offerList = new List<Offer>()
{
new Offer() { SupplierID = 1, ProductID = 2},
new Offer() { SupplierID = 2, ProductID = 1},
new Offer() { SupplierID = 2, ProductID = 3}
};
If you want to show names of suppliers whiches products have been offered then your LINQ query should be as this:
IEnumerable<string> result = from supplier in supplierList
join offer in offerList on supplier.SupplierID equals offer.SupplierID
join product in productList on offer.ProductID equals product.ProductID
where supplier.City == "Chicago"
group supplier by supplier.SuppierName into g
select g.Key;
You can see if correct names have been selected:
foreach (string supplierName in result)
{
Console.WriteLine(supplierName);
}
It must give following result:
FirstCompany
SecondCompany

Answer 2

You could try this:
var query = from s in Supplier
join o in Offers on s.Supp_ID equals o.Supp_ID
join p in Product on o.Prod_ID equals p.Prod_ID
where s.City == "Chicago"
group s
by new {s.City, s.Name} //added this
into Results
select new { Name = Results.Key.Name };

Answer 3

You group s (Supplier) by s.City. The result of this is an IGrouping<City, Supplier>. I.e. only City and Supplier are within reach after the grouping: for each City you get an IEnumerable<Supplier> of its suppliers (which will be multiplied by the joins, by the way).
Since you also have the condition where s.City == "Chicago" grouping by city is of no use. There is only one city. So I think you may as well do something like this:
from s in Supplier
join o in Offers on s.Supp_ID equals o.Supp_ID
join p in Product on o.Prod_ID equals p.Prod_ID
where s.City == "Chicago"
select new {
City = s.City.Name,
Supplier = s.Name,
Product = p.Name,
...
};