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CREATE VIEW view-name ( column-name ) ,
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FIGURE 14-2
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The CREATE VIEW statement syntax diagram
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specified; the data type, length, and other characteristics of each column are derived from the definition of the columns in the source tables. If the list of column names is omitted from the CREATE VIEW statement, each column in the view takes the name of the corresponding column in the query. The list of column names must be specified if the query produces two columns with identical names and in some DBMS products, if the query includes calculated columns. While some DBMS products automatically assign column names to calculated columns, the names are usually not very useful, so it s a much better practice to always assign column names to calculated columns. Although all views are created in the same way, in practice, different types of views are typically used for different purposes. The next few sections examine these types of views and give examples of the CREATE VIEW statement.
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Horizontal Views
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A common use of views is to restrict a user s access to only selected rows of a table. For example, in the sample database, you may want to let a sales manager see only the SALESREPS rows for salespeople in the manager s own region. To accomplish this, you can define two views, as follows: 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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PART IV
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Create a view showing Western region salespeople.
CREATE VIEW SELECT FROM WHERE WESTREPS AS * SALESREPS REP_OFFICE IN (21, 22);
Now you can give each sales manager permission to access either the EASTREPS or the WESTREPS view, denying them permission to access the other view and the SALESREPS table itself. This effectively gives the sales manager a customized view of the SALESREPS table, showing only salespeople in the appropriate region. A view like EASTREPS or WESTREPS is often called a horizontal view. As shown in Figure 14-3, a horizontal view slices the source table horizontally to create the view. All of the columns of the source table participate in the view, but only some of its rows are visible through the view. Horizontal views are appropriate when the source table contains data
Part IV:
Database Structure
EASTREPS View EMPL_NUM NAME 105 109 106 104 101 103 Bill Adams Mary Jones Sam Clark Bob Smith Dan Roberts Paul Cruz
AGE 37 31 52 33 45 29
SALESREPS Table EMPL_NUM NAME 105 109 102 106 104 101 110 108 103 107 Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch Paul Cruz Nancy Angelli
AGE 37 31 48 52 33 45 41 62 29 49
SALES $367,911.00 $392,725.00 $474,050.00 $299,912.00 $142,594.00 $305,673.00 $75,985.00 $361,865.00 $286,775.00 $186,042.00
WESTREPS View EMPL_NUM NAME 102 Sue Smith 108 Larry Fitch 107 Nancy Angelli
AGE 48 62 49
FIGURE 14-3
Two horizontal views of the SALESREPS table
that relates to various organizations or users. They provide a private table for each user, composed only of the rows needed by that user. Here are some more examples of horizontal views: De ne a view containing only Eastern region of ces.
CREATE VIEW SELECT FROM WHERE EASTOFFICES AS * OFFICES REGION = 'Eastern';
De ne a view for Sue Smith (employee number 102) containing only orders placed by customers assigned to her.
CREATE VIEW SELECT FROM WHERE SUEORDERS AS * ORDERS CUST IN (SELECT CUST_NUM FROM CUSTOMERS WHERE CUST_REP = 102);
De ne a view showing only those customers who have more than $30,000 worth of orders currently on the books.
CREATE VIEW SELECT FROM WHERE BIGCUSTOMERS AS * CUSTOMERS 30000.00 < (SELECT SUM(AMOUNT) FROM ORDERS WHERE CUST = CUST_NUM);
14:
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In each of these examples, the view is derived from a single source table. The view is defined by a SELECT * query and therefore has exactly the same columns as the source table. The WHERE clause determines which rows of the source table are visible in the view.
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