From experience the questions from the Online QB are randomly picked from the database, but do give a pretty accurate picture of what is in the exams..
You will find though that the same question can turn up more than once in the database, so while some subjects may have 600 questions quite a few could be repeats, just worded slightly different etc.
The best way is to do a sequencial exam questions 1-100 and then the 100-200 etc until all are covered and you're happy with them.