SQL and Relational Theory

Appendix C. Answers to Exercises

Chapter 1

Exercise 1-1. Here are a few examples of statements from the early part of the chapter where the term relation should be replaced by the term relvar:

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

• “A foreign key is a set of attributes in one relation whose values are required to match the values of some candidate key in some other relation (or possibly the same relation).”

• “[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.

Exercise 1-2. E. F. Codd (1923–2003) was the original 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 http://www.acm.org/sigmod/ and elsewhere. An expanded version of that tribute appears in my book Date on Database: Writings 2000-2006 (Apress, 2006).

Exercise 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). Domain is the term used in most of the older database literature, ...