This section attempts to outline to what extent PostgreSQL conforms to the SQL standard. Full compliance to the standard or a complete statement about the compliance to the standard is complicated and not particularly useful, so this section can only give an overview. The formal name of the SQL standard is ISO/IEC 9075 Database Language SQL. A revised version of the standard is released from time to time; the most recent one appearing in 1999. That version is referred to as ISO/IEC 9075:1999, or informally as SQL99. The version prior to that was SQL92. PostgreSQL development tends to aim for conformance with the latest official version of the standard where such conformance does not contradict traditional features or common sense. At the time of this writing, balloting is under way for a new revision of the standard, which, if approved, will eventually become the conformance target for future PostgreSQL development. SQL92 defined three feature sets for conformance: Entry, Intermediate, and Full. Most database management systems claiming SQL standard conformance were conforming at only the Entry level, since the entire set of features in the Intermediate and Full levels was either too voluminous or in conflict with legacy behaviors. SQL99 defines a large set of individual features rather than the ineffectively broad three levels found in SQL92. A large subset of these features represents the core features, which every conforming SQL implementation must supply. The rest of the features are purely optional. Some optional features are grouped together to form packages, which SQL implementations can claim conformance to, thus claiming conformance to particular groups of features. The SQL99 standard is also split into 5 parts: Framework, Foundation, Call Level Interface, Persistent Stored Modules, and Host Language Bindings. PostgreSQL only covers parts 1, 2, and 5. Part 3 is similar to the ODBC interface, and part 4 is similar to the PL/pgSQL programming language, but exact conformance is not specifically intended in either case. In the following two sections, we provide a list of those features that PostgreSQL supports, followed by a list of the features defined in SQL99 which are not yet supported in PostgreSQL. Both of these lists are approximate: There may be minor details that are nonconforming for a feature that is listed as supported, and large parts of an unsupported feature may in fact be implemented. The main body of the documentation always contains the most accurate information about what does and does not work. Feature codes containing a hyphen are subfeatures. Therefore, if a particular subfeature is not supported, the main feature is listed as unsupported even if some other subfeatures are supported. Identifier Package Description Comment &features-supported; The following features defined in SQL99 are not implemented in this release of PostgreSQL. In a few cases, equivalent functionality is available. Identifier Package Description Comment &features-unsupported;