And also ...

In response to Andrew C-S, who said that EF Codd's "abstract mathematical concept [of a relation] corresponds closely to the table in modern databases" ... well, sadly, it doesn't. Relations can't have duplicates (for two reasons; relations are sets, and sets can't have duplicates; and relations form assertions of fact, so it's pointless asserting the same fact twice in the absence of "fuzzy" or weighted logics). Also, as originally defined, relations don't allow null.

SQL tables both allow duplicates and allow nulls, so strictly aren't relations (although if you're sensible, you'll always design your tables to prevent duplicates, and avoid using null because of tri-value logic horrors).

- Tony

In a world without walls or fences, what need have we of Windows or Gates ?