LinQ

Cross Join

    var q = 
        from c in categories 
        join p in products on c equals p.Category 
        select new { Category = c, p.ProductName };  

Group Join

   var q = 
        from c in categories 
        join p in products on c equals p.Category into ps 
        select new { Category = c, Products = ps };  

Left Outer Join

    var q = 
        from c in categories 
        join p in products on c equals p.Category into ps 
        from p in ps.DefaultIfEmpty() 
        select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName };