Thanks Mark for your help.

I am not sure I understand 100% what you mean but I will think about it.

I bought the book yesterday Glad to hear another recommendation...

You are absolutely right, I would use -1 but I've written not null so that is incorrect. I love the -2 idea, I was just going to use -1 for...

Hi Mark,

thanks for your reply

Payments can be related to an order or an account/customer, that's the reason order_id and customer_id can be NULL as it is one or the...