Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 2 SQL AND PL/SQL 37
Listing 2-18. A Check Constraint
In the preceding scenario, how many records are retrieved by the SQL query in
Listing 2-19?
Listing 2-19. An SQL Query with Counterintuitive Results
Intuitively, we expect to retrieve all rows in the table, because the only valid values of
are and and because that rule is enforced by a check constraint. However,
Oracle will nevertheless retrieve only two rows. This is not a sensible result.
Counterintuitively, the queries in Listing 2-20 also retrieve only two data records.
Listing 2-20. Two More Examples of SQL Queries with Counterintuitive Results
The reasoning offered for these counterintuitive results is that the status of TOOL
TIME, INC. is unknown and, therefore, the answer to every question involving the status
of TOOL TIME, INC. is also unknown. To retrieve the third record, you need to add the
clause to the preceding SQL queries; you can find more information
on the problems created by nullable data items in any good book on SQL.