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However, for the human user of the database, it s much easier to write and understand the single-table query that references the view.
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What does it mean to insert a row of data into a view, delete a row from a view, or update a row of a view For some views, these operations can obviously be translated into equivalent operations against the source table(s) of the view. For example, consider once again the EASTREPS view, defined earlier in this chapter: Create a view showing Eastern region salespeople.
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CREATE VIEW SELECT FROM WHERE EASTREPS AS * SALESREPS REP_OFFICE IN (11, 12, 13)
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This is a straightforward horizontal view, derived from a single source table. As shown in Figure 14-5, it makes sense to talk about inserting a row into this view; it means the new row should be inserted into the underlying SALESREPS table from which the view is derived. It also makes sense to delete a row from the EASTREPS view; this would delete the corresponding row from the SALESREPS table. Finally, updating a row of the EASTREPS view makes sense; this would update the corresponding row of the SALESREPS table. In each case, the action can be carried out against the corresponding row of the source table, preserving the integrity of both the source table and the view. However, consider the ORD_BY_REP grouped view, as it was defined earlier in the section Grouped Views :
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Figure 14-5.
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Define a view that contains summary order data for each salesperson.
CREATE VIEW ORD_BY_REP (WHO, HOW_MANY, TOTAL, LOW, HIGH, AVERAGE) AS SELECT REP, COUNT(*), SUM(AMOUNT), MIN(AMOUNT), MAX(AMOUNT), AVG(AMOUNT) FROM ORDERS GROUP BY REP
There is no one-to-one correspondence between the rows of this view and the rows of the underlying ORDERS table, so it makes no sense to talk about inserting, deleting, or updating rows of this view. The ORD_BY_REP view is not updateable; it is a read-only view. The EASTREPS view and the ORD_BY_REP view are two extreme examples in terms of the complexity of their definitions. There are views more complex than EASTREPS where it still makes sense to update the view, and there are views less complex than ORD_BY_REP where updates do not make sense. In fact, which views can be updated and which cannot has been an important relational database research problem over the years.
View Updates and the ANSI/ISO Standard
The ANSI/ISO SQL1 standard specifies the views that must be updateable in a database that claims conformance to the standard. Under the standard, a view can be updated if the query that defines the view meets all of these restrictions: I DISTINCT must not be specified; that is, duplicate rows must not be eliminated from the query results. I The FROM clause must specify only one updateable table; that is, the view must have a single source table for which the user has the required privileges. If the source table is itself a view, then that view must meet these criteria. I Each select item must be a simple column reference; the select list cannot contain expressions, calculated columns, or column functions. I The WHERE clause must not include a subquery; only simple row-by-row search conditions may appear. I The query must not include a GROUP BY or a HAVING clause. The basic concept behind the restrictions is easier to remember than the rules themselves. For a view to be updateable, the DBMS must be able to trace any row of the view back to its source row in the source table. Similarly, the DBMS must be able to trace each individual column to be updated back to its source column in the source table.
14:
Views
If the view meets this test, then it s possible to define meaningful INSERT, DELETE, and UPDATE operations for the view in terms of the source table(s).
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