3.10 READINGS

Information on the Public Petroleum Data Model can be found at www.ppdm.org.

Datetime literals are based on an ISO standard, "Representation of Dates and Times" [43]. This standard uses the Gregorian calendar as well as a 24-hour clock, which also serve as the basis for SQL datetimes.

While the SQL-92 standard [44] is quite lengthy, at 580 pages, only a small portion, about 30 pages, or 5 percent, concerns temporal data types and their operators. However, this portion in some ways is more complex than other parts of SQL, as evidenced by over 12 pages (almost 10 percent) of the Technical Corrigendum 3 [19]. Even with these numerous corrections, many of the deficiencies discussed in Section 3.7 remain. Sykes provides a cogent discussion of the problems, and partial solutions, to time zone support in SQL-92 [101].

The temporal constructs are included in the Intermediate SQL and Full SQL levels of conformance to SQL-92; the Entry SQL level of conformance includes no temporal types. Conformance testing was initially done by the National Institute of Standards and Technology (NIST), a U.S. Department of Commerce agency. As of July 1, 1997, when NIST ceased SQL conformance testing, 11 products had been validated for conformance to FIPS publication 127-2 [73]: IBM (2 configurations), Informix (5 configurations), NCR, and Sybase (3 configurations). Unfortunately, all of these validations were at Entry FIPS 127-2, which includes no time support. The National Software Testing Laboratories (NSTL), an independent organization not associated with the government, has established...