SQL1999

SQL1999, also known as SQL:1999 or SQL3, is a major revision to the SQL standard issued by ISO/IEC in 1999. It followed SQL-92 and extended SQL to incorporate object-relational database features as part of the ISO standard for SQL. It defined a broad set of capabilities intended to standardize both traditional relational queries and more advanced data modeling.

Key additions include object-relational features: the ability to define user-defined structured types, distinct types and array types, as well as reference types. A subtype hierarchy can be declared using CREATE TYPE ... UNDER, and methods can be attached to types; the standard also defines mechanisms for referencing objects (REF) and for storing object data in tables.

Procedural extensions: SQL/PSM provides a persistent stored modules language for writing stored procedures and functions, with variables, control structures, cursors, and exception handling. Triggers were introduced to support automatic execution in response to data modification events.

Query and representation enhancements: the WITH clause to define common table expressions (including recursive forms), enhanced support for large objects, and extended domain and constraint capabilities. The standard also formalized support for object-relational data types and operations within SQL.

Impact: SQL:1999 represented a shift towards object-relational database features and laid groundwork for later standards such as SQL:2003 and beyond. Real-world adoption varied, with some systems implementing only subsets of its features while others extended their products to align with the new capabilities.