SQL Conformance Levels

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

SQL-92 defines three conformance levels: entry, intermediate and full. Conformance claims must state to which level they are claiming conformance1. Most database just conform to entry-level2.

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.

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  1. Unproven claim for dramatic effect. However, in his paper “Is SQL a real standard anymore” the former secretary of the ANSI committee on databases writes that implementing Core SQL:1999 “is close to impossible for all vendors except for two of three.”

  2. SQL-92, §23.2

  3. According to my observations but also stated in the PostgreSQL documentation.

  4. “SQL in a Nutshell” by Kevin Kline [ISBN: 978-0-596-51884-4].

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