Issues with sequential number

Jorg provided me with this formula a while ago to create an auto-incrementing order number.

let myyear := format(today(), "YY");
let myOrderNr := max((select Purchases where substr('Order number', 3, 2) = myyear).number(substr('Order number', 6)));
if cnt(myOrderNr) < 1 then
'Order number' := text("OR_" + number(format(today(), "YY")) + "_0001")
else
'Order number' := text("OR_" + number(format(today(), "YY")) + "_" + format(myOrderNr + 1, "0000"))

I'm now trying to create a similar formula. Format is STATIC TEXT_ORDER DATE_INCREMENTING NUMBER

so

BLA_190625_001

The date is being pulled from a composite table, though I also have a formula pulling in the date in the table where this reference number is to be. I'm just having trouble wrapping my head around the concept, I don't know why. I understand I need to use the max()+1 string to look at the previous records number and so on...

I just can't even begin to process it haha.