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7.3 Rules on INSERT, UPDATE, and DELETE

Rules that are defined on INSERT, UPDATE, and DELETE are significantly different from the view rules described in the previous section. First, their CREATE RULE command allows more:

• They are allowed to have no action.
• They can have multiple actions.
• They can be INSTEAD or ALSO (the default).
• The pseudorelations NEW and OLD become useful.
• They can have rule qualifications.

Second, they don't modify the query tree in place. Instead they create zero or more new query trees and can throw away the original one.