SQL and Relational Theory, 2nd Edition

CHAPTER 1

1.1 Here are a few examples of statements from the early part of the chapter in which every occurrence of the term relation (highlighted here in bold) should be replaced by the term relvar:

• “[Every] relation has at least one candidate key.”

• “[A] foreign key is a combination, or set, of attributes FK in some relation r2 such that each FK value is required to be equal to some value of some key K in some relation r1 (r1and r2 not necessarily distinct).”

• “[The] relational assignment operator ... allows the value of some relational expression ... to be assigned to some relation.”

• “A view (also known as a virtual relation) is a named relation whose value at any given time t is the result of evaluating a certain relational expression at that time t.”

And so on.

1.2 E. F. Codd (1923–2003) was the inventor of the relational model, among many other things. In December 2003 I published a brief tribute to him and his achievements, which you can find on the ACM SIGMOD website www.acm.org/sigmod and elsewhere. Note: An expanded version of that tribute appears in my book Date on Database: Writings 2000–2006 (Apress, 2006).

1.3 A domain can be thought of as a conceptual pool of values from which actual attributes in actual relations take their actual values. In other words, a domain is a type, and the terms domain and type are effectively interchangeable—but personally I much prefer type, as having a longer pedigree (in the computing world, at least), as well as being slightly more succinct. ...