http://db.cs.berkeley.edu/papers/ERL-M90-36.pdf
a simple rule system supports a powerful view system in current commercial system. Not only can views by specified by using rules but also special semantics for resolving ambiguous view updates as simply additional rules. … it subsumes both views and procedures as special case.
… all queries and many updates to … can be mapped to commands on the underlying base relations, … the ability to specify update semantics for ambiguous updates to views would be a desirable enhancement.
The semantics of a PRS2 rule is that at the time an individual tuple is accessed, updated, inserted, or deleted, there is a CURRENT tuple (for retries, replace and deletes) and a NEW tuple (for replaces and appends). If the event and condition specified in the ON clause are true for the CURRENT tuple, then the action part of the rule is executed.