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Databases - Practical PostgreSQL
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SQL Standards

The American National Standards Institute (ANSI) standardized SQL in 1986 (X3.135) and the International Standards Organization (ISO) standardized it in 1987. The United States government's Federal Information Processing Standard ( FIPS ) adopted the ANSI/ISO standard. In 1989, a revised standard known commonly as SQL89 or SQL1 , was published.

Due partially to conflicting interests from commercial vendors, much of the SQL89 standard was intentionally left incomplete, and many features were labeled implementor-defined. In order to strengthen the standard, the ANSI committee revised its previous work with the SQL92 standard ratified in 1992 (also called SQL2 ). This standard addressed several weaknesses in SQL89 and set forth conceptual SQL features which at that time exceeded the capabilities of any existing RDBMS implementation. In fact, the SQL92 standard was approximately six times the length of its predecessor. As a result of this disparity, the authors defined three levels of SQL92 compliance: Entry-level conformance (only the barest improvements to SQL89), Intermediate-level conformance (a generally achievable set of major advancements), and Full conformance (total compliance with the SQL92 features).

More recently, in 1999, the ANSI/ISO released the SQL99 standard (also called SQL3 ). This standard addresses some of the more advanced and previously ignored areas of modern SQL systems, such as object-relational database concepts, call level interfaces, and integrity management. SQL99 replaces the SQL92 levels of compliance with its own degrees of conformance: Core SQL99 and Enhanced SQL99 .

PostgreSQL presently conforms to most of the Entry-level SQL92 standard, as well as many of the Intermediate- and Full-level features. Additionally, many of the features new in SQL99 are quite similar to the object-relational concepts pioneered by PostgreSQL (arrays, functions, and inheritance).

Databases - Practical PostgreSQL
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