Re: Not sure where to start with MySQL

[Patrick Arendse]

Hi Peter

As per your advice I clicked on Introduction to databases and started reading Relational Databases which I see is authored by yourself and Arthur Fuller. All went well up to a point when I got stuck. Something did not make sense to me. So I hope you do not mind me asking as I do not like proceeding without full understanding. Maybe it was a typo or maybe my lack of understanding (more likely the latter!).

Anyway, under the heading, Some basic facts about sets, there is a line:

Given two sets A and B, the members that belong to A and/or B constitute the union of A and B—in Fig 2, M-P-C + C-M-P + M&C as per Fig. 2.

Now if set A = Men, and set B = Crooks then to me the Union of A and B = M-P-C + M&C + C-M-P + M&P + M&C&P + C&P. I thus get 3 more terms because is M&P and M&P&C not part of A and C&P also not part of B? Why were these left out in the union expression?

Thanks.