How does the Count function work in a LINQ left join-CodePudding

So I have this example of a LINQ Query:

var productsPerSupplier = from s in suppliers
                          join p in products
                          on s.Id equals p.SupplierId
                          into sp
                          select new
                          {
                              SupplierId = s.Id,
                              SupplierName = s.Name,
                              ProductCount = sp.Count()
                          };

How does the count function know to count the number of products for each supplier? What is the structure of sp table? Isn't it supposed to count all the rows in the table?

This is the result:

CodePudding user response：

See here in the docs. into keyword creates hierarchical grouping in the results where each item in the left table is associated with one or more items in the right table of your join. This is why it knows how to perform Count() on individual groups.

CodePudding user response：

Try following :

var productsPerSupplier = (from s in suppliers
                          join p in products
                          on s.Id equals p.SupplierId
                          select new {s = s, p = p}
                          ).GroupBy(x => x.s.ID)
                          .Select(x => new
                          {
                              SupplierId = x.Key,
                              SupplierName = x.s.First().Name,
                              ProductCount = x.Count()
                          }).ToList();