Database

Modifying Views

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

Constraints and Triggers

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

Keys Declared With UNIQUE

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

Modifications by Cursor

When a cursor ranges over the tuples of a base table (i.e., a relation that is stored in the database, rather than a view or a relation constructed by a query), then one can not only read and process the value of each tuple, but one can update or delete tuples. The syntax of

Protecting Against Concurrent Updates

Assume that as we observe the net worths of movie executives using the function worthRanges of Cursors Figure 1, some other process is modifying the underlying MovieExec relation. We shall have more to say about a number of processes accessing a single database at the

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