Background

I'm using 4D Database (v17.5) and trying to query records. I have the following simplified schema:

Customers (table)
    -CustomerID (field)
    -CompanyName (field)
    -City (field)
    -State (field)

SalesOrders (table)
    -CustomerID (field)
    -OrderTotal (field)

They are tied together with a Many to One automatic relation:

[Customers]CustomerID (O)<-------------------------(M) [SalesOrders]CustomerID
            (ParentCustomer)              (ChildSalesOrders)

I can query to get a set of [SalesOrders] records using either of the following:

ds.SalesOrders.query("OrderTotal > 500 AND ParentCustomer.State = 'New York'")
ds.Customers.query("State = 'New York' AND ChildSalesOrders.OrderTotal > 500").ChildSalesOrders

Problem

Some customers have no orders at all, and if I want to find a set of customers with zero related orders, I am having trouble.

I have gotten the following to work:

$voCustomersToCheck:=ds.Customers.query("State = 'New York')
$voCustomersWithoutOrders:=ds.Customers.newSelection()

For each($voCustomer;$voCustomersToCheck)
    If ( $voCustomer.ChildSalesOrders.length > 0 )
        $voCustomersWithoutOrders.add($voCustomer)
    End if
End for each

What I'd like to do instead is something like:

$voCustomersWithoutOrders:=ds.Customers.query("State = 'New York' AND ChildSalesOrders.length = 0')

That doesn't work, though.

Question

Is what I'm trying to accomplish possible?

In v17.5, it is impossible.

That said, if you update to v18 or higher, you can use the NULL keyword; so the solution is to write:

$voCustomersWithoutOrders:=ds.Customers.query("State = 'New York' AND ChildSalesOrders = NULL')