In limited conditions it is possible to carry out an insertion, deletion, or update to a view. In the beginning, this idea makes no sense at all, since the view does not exist the way a base table (stored relation) does. What could it mean, say, to insert a new tuple into a view? Where

In this section we shall study those aspects of SQL that let us create "active" elements. An active element is an expression or statement that we write once, store in the database, and expect the element to execute at appropriate times. The time of action might be when a certain

One more way to declare a key is to use the keyword UNIQUE. This word can appear exactly where PRIMARY KEY can appear: either following an attribute and its type or as a separate item within a CREATE TABLE statement. The meaning of a UNIQUE declaration is nearly the

A second important kind of constraint on a database schema is that values for certain attributes must make sense. That is, an attribute like presC# of relation Studio is expected to refer to a particular movie executive. The implied "referential integrity" constraint is that if a studio's

We have seen key constraints, which compel certain attributes to have distinct values among all the tuples of a relation, and we have seen foreign-key constraints, which enforce referential integrity between attributes of two relations. Now, we shall see a third important kind of

To declare a constraint on the tuples of a single table R, when we define that table with a CREATE TABLE statement we may add to the list of attributes and key or foreign-key declarations the keyword CHECK followed by a parenthesized condition. This condition can be anything that

The most powerful forms of active elements in SQL are not associated with specific tuples or components of tuples. These elements, called "triggers" and "assertions," are part of the database schema, on a par with the relations and views themselves.

There is a useful feature of triggers that did not make the SQL-99 standard, but figured into the discussion of the standard and is supported by some commercial systems. This extension allows BEFORE or AFTER to be replaced by INSTEAD OF; the meaning is that when an event