The SQL standard is huge. More than 4000 pages in its SQL:2011 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
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 features3.
Beyond Core SQL, vendors can claim conformance on a feature-by-feature basis.