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As described in 1, table views allow you to combine columns from multiple tables into a virtual table for querying The combination of columns is performed as the result of a query using the SELECT command This feature allows you to create a result set and use it just as if it were a physical table This is demonstrated in Figure 7-1 Views are often used to help simplify complex sub-selects within SELECT commands Instead of constantly having to type a complex sub-select, you can assign it to a view, then use the view in your SELECT commands Views are created using the CREATE VIEW SQL command The format of this command is
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CREATE [ OR REPLACE ] [ TEMP | TEMPROARY ] VIEW viewname [ (column_name,] ) ] AS query
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PostgreSQL 8 for Windows
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Customer ID Last name First name Product ID
Product name Quantity Cost
Customer orders view
Figure 7-1 Creating a view from table columns
The CREATE OR REPLACE VIEW command allows you to overwrite an existing view When you do this, the column names and data types for the new view must match the column names and data types of the existing view By default, PostgreSQL creates the view in the default schema within the active database You can save the view in another schema within the database by using the full schemanameviewname format for the viewname parameter Remember, to create a new view in a schema, the Login Role must have CREATE privileges in the schema The superuser can grant this privilege to trusted users Once the view is created, the view owner can assign privileges to the view to Group Roles, just like a normal table object Alternatively, if you do not need to create a permanent view, you can use the TEMP or TEMPORARY keyword This makes the view available only within the current session Once you quit the session, PostgreSQL automatically drops the view By default, PostgreSQL assigns column names to the view columns based on the result set columns names of the specified query You can alter the column names by including the column name list column_name in the command Here is an example of creating a view based on a complex SELECT command:
test=# create view store"CustomerOrders" AS test-# select "Customer""LastName", "Customer""FirstName",
7:
Using Advanced SQL
test-# test-# test-# CREATE test=# GRANT test=#
"Product""ProductName", "Order""TotalCost" from store"Order" natural inner join store"Customer" natural inner join store"Product"; VIEW grant select on store"CustomerOrders" to "Salesman";
After creating the view (remember, you need the proper privileges to do this) and assigning privileges to allow Group Roles to use it, other users can query the view just as if it were a table PostgreSQL only allows you to query views As of the time of this writing, you are not able to insert, remove, or modify data directly from a view To display the views available in a database, use the \dv meta-command (plus the schema name if the view was created in specific schema):
test=> \dv store List of relations Schema | Name | Type | Owner + + + store | CustomerOrders | view | postgres (1 row) test=>
Once created, users can query the view just as if it were a normal table:
test=> select * from store"CustomerOrders"; LastName | FirstName | ProductName | TotalCost + + + Blum | Rich | Laptop | $5,00000 Blum | Barbara | Laptop | $1,00000 Blum | Katie | Desktop | $30000 (3 rows) test=>
If you no longer need the view, the view owner can remove it using the DROP VIEW command
TABLE INDEXES
Querying tables is the most computer-intensive part of using a database For each query, PostgreSQL must compare each record contained in the table with any conditions defined in the SELECT command This can consume both time and processor power, especially when working with large amounts of data
PostgreSQL 8 for Windows
Customer table
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