Establishing Table Structures - Database Design for Mere Mortals - page 167

Figure 7.16. An example of a “hidden” multipart field
Resolving Multivalued Fields
Asyouknow,amultivaluedfieldcanpotentiallystoretwoormoreoccurrencesofthe same
value.Fortunately,you'llrecognizeamultivaluedfieldwhenyouseeone.Thefield'sname
is often plural and its value almost invariably contains a number of commas, which serve
to separate the various occurrences that exist within the value itself.
Resolving multipart fields is not very hard at all, but resolving multivalued fields can be
a little more difficult and will take some work. A multivalued field has the same funda-
mental set of problems as a multipart field, as the C ATEGORIES T AUGHT field in Figure 7.17
clearly illustrates. For example, you'll have difficultly retrieving information for everyone
who teaches a specific category (such as WP), you can't sort the data in any meaningful
fashion,and,mostimportant,youdon'thaveroomtoentermorethanfourcategories.What
happens when one or more instructors teach five categories? The only option you'll have
is to make the field larger every time you need to enter more values than it will currently
allow.