LINQ系列：Linq to Object分组操作符

public static IQueryable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector);

var expr = from p in context.Products
            group p by p.CategoryID;

foreach (var item in expr)
{
    foreach (var p in item)
    {
        Console.WriteLine(p.ProductName);
    }
}

var expr = context.Cities.GroupBy(c => c.ProvinceID);