Question

In a previous question I asked how I would get a Customers first Order, it was answered thus :

var minOrders = from customer in DataSet.Customers                 
let order = (from o in DataSet.Orders where o.CustomerId == customer.CustomerId
order by o.OrderTimestamp                              
select o).first()
select new {
    customer.Name,
    order.OrderAmount
}); 

This is great, but how do I include a Left Outer Join onto the above? That is, return all Customers even if they have no orders, so something like :

var minOrders = from customer in DataSet.Customers LEFT OUTER JOIN
let order = (from o in DataSet.Orders where o.CustomerId == customer.CustomerId
order by o.OrderTimestamp                              
select o).first()
select new {
    customer.Name,
    order.OrderAmount
});

I know, in hindsight I should of asked this at the same time..

Thanks, Joe

Answer 1

Firstly, using let to do the join like this isn’t ideal in the first place. There’s a join clause in LINQ for a reason 🙂

Left outer joins aren’t specifically supported in LINQ, but you can fake them like this:

var minOrders = from customer in DataSet.Customers                 
                join order in DataSet.Orders.OrderBy(o => o.OrderTimestamp)
                     on customer.CustomerId equals o.CustomerId
                     into customerOrders
                let order = customerOrders.FirstOrDefault()
                select new {
                    customer.Name,
                    OrderAmount = order == null ? 0m : order.OrderAmount
                };

Usually a left outer join uses from foo in bar.DefaultIfEmpty instead of let foo = bar.FirstOrDefault() but in this case you’re only after the first match anyway, hence the different approach.

I’m pretty sure this works logically – whether the SQL translation will work or not is a different matter.