Declarative constraints and triggers are two essential features that have been introduced to support active data requirements in relational database systems. Given their differing expressive powers, support for both is required. The semantics of this interaction must be carefully defined to avoid inconsistent execution and to provide users a comprehensive model for understanding such interaction. In this talk I will describe a model for procedural triggers in SQL. This model integrates trigger execution with the set semantics for evaluating declarative constraints that is defined by the SQL international standard (SQL2) and implemented by most commercial SQL database vendors. Our model was implemented in DB2 for common servers and has recently been proposed and accepted as the model for the emerging new version of the SQL standard (SQL3).
A paper on this topic is available upon request from the speaker or the seminar organizer.