So we have a grid hierarchy of Orders and then order item.
Now if I want to add an item to an order that is simple enough with using a dropdowncolumn.
select * from products ( for example).
Now here is were it gets a bit harder. Each customer has its own discrete list of items it may order.
How do I create a query that select items and uses the customer id in the order as part of the selection ?
select * from products where customer.id=X

I don't see anyone else responding. So here's what I suspect; This is the code example for the three level grid. Does this sound like what you are asking?
This is how the orders table is linked to the customers table.
$table_order->AddRelationField("customerNumber","customerNumber"); <-- this gets added to the SQL query (select and where clauses)
$table_order->AutoGenerateExpandColumn = true;
$table_order->AutoGenerateColumns = true;
$table_order->DisableAutoGenerateDataFields = "customerNumber"; <-- this tells the lower level grid to leave out this column on the grid even though it is in the result set
$table_order->AddDetailTable($table_order_detail);

Not exactly. The code you listed I believe shows how to link parent child grids.
What I am getting at is in the child grid I want to add or edit a row in that in that row is droplist column.
Said droplist is generated by a sql command and then a foreach loop to add in the result set as items for the drop list.
The trick I am looking for is when coding in the sql how to I include the customer id from the parent grid.

It sounds like an edit template can be used instead. That way when you attempt to add/edit information for a specific customer/user/etc..., you may customize your form and post back any data you need as long as it can be posted. I use edit templates all of the time to do all kinds of nice saves. I hope this helps.

Anthony is right. The best solution is to use GridTemplate.