The SQL standard is huge. More than 4300 pages in its SQL:2023 incarnation. No single implementation can ever implement all features.0 Even in the early releases, such as SQL-92, the SQL standard defined different conformance levels so that vendors can claim conformance to a subset of the standard.
Conformance Levels in SQL-92
SQL-92 defines three conformance levels: entry, intermediate and full. Conformance claims must state to which level they are claiming conformance.1 Most database just conform to entry-level.2
Optional Features since SQL:1999
Starting with SQL:1999 all features are enumerated and either flagged mandatory or optional. As a bare minimum, conforming systems must comply with all mandatory features, which are collectively called “Core SQL”. Besides entry-level SQL-92 features, Core SQL:1999 also requires some features previously only required for intermediate or full level as well as a few new features.3
Beyond Core SQL, vendors can claim conformance on a feature-by-feature basis.
