I can't see the comments on that site (maybe adblock is killing them?), so this may be a duplicate, but I would suggest reading Date's "SQL and Relational Theor

I can't see the comments on that site (maybe adblock is killing them?), so this may be a duplicate, but I would suggest reading Date's "SQL and Relational Theory" and looking at Tutorial D (an alternative to SQL) before implementing this. At the very least it would clarify how you might use sets and where you need to worry about things like ordering if you are going to reproduce SQL faithfully.