SQL in a Nutshell, 3rd Edition

Levels of Conformance

SQL99 is built upon SQL92’s levels of conformance. SQL92 first introduced levels of conformance by defining three categories: Entry, Intermediate, and Full. Vendors had to achieve at least Entry-level conformance to claim ANSI SQL compliance. The U.S. National Institute of Standards and Technology (NIST) later added the Transitional level between the Entry and Intermediate levels, so NIST’s levels of conformance were Entry, Transitional, Intermediate, and Full, while ANSI’s were only Entry, Intermediate, and Full. Each higher level of the standard was a superset of the subordinate level, meaning that each higher level included all the features of the lower levels of conformance.

Later, SQL99 altered the base levels of conformance, doing away with the Entry, Intermediate, and Full levels. With SQL99, vendors must implement all the features of the lowest level of conformance, Core SQL99, in order to claim (and publish) that they are SQL99 ready. Core SQL99 includes the old Entry SQL92 feature set, features from other SQL92 levels, and some brand new features. A vendor may also choose to implement additional feature packages described in the SQL99 standard.