Support zero/optional relationship in DBML

Right now it is not possible to visualize whether a relationship is mandatory or optional. As these options are very common, this feature would allow us to create more detailed and complete schema diagrams.

For example, to show that details are optional for an order, we could include a new symbol “0>”:

Table order_detail {
  orderId int [ref: 0> order.orderId]
  detailDescription varchar

With this, we communicate an order may have 0 or many order_details, but every order_detail must only relate to one order.
order_detail >0------|| order

To show that the other side is optional we could put the 0 on the other side of the operator “>0”.