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This chapter is from the book

View Commands

Like tables, views have CREATE and DROP commands. (DROP VIEW applies only to the view, not to the underlying base table.) You access the data with SELECT statements.

Creating Views

Here's the simplified syntax of a view definition statement:

SYNTAX

CREATE VIEW view_name [(column_name [, column_name]...)]
AS
SELECT_statement
  • Follow your system's rules for identifiers when you name the view.

  • Specify view column names if the column is calculated or if multiple columns have the same name. Otherwise, columns inherit names from the SELECT clause.

  • Define the columns and rows of the view in the SELECT statement.

The following example creates a view that displays the names of authors who live in Oakland, California, and their books. The view is named oaklanders. Its SELECT statement pulls data from three tables.

SQL
create view oaklanders (FirstName, LastName, Book)
as
select au_fname, au_lname, title
from authors, titles, titleauthors
where authors.au_id = titleauthors.au_id
 and titles.title_id = titleauthors.title_id
 and city = 'Oakland'

As you've seen, the SELECT statement doesn't need to be a simple selection of the rows and columns of one particular table. You can create a view using any of these objects or combinations of objects:

  • A single table
  • More than one table
  • Another view
  • Multiple views
  • Combinations of views and tables

The SELECT statement can have almost any complexity, using projection and selection to define the columns and rows you want to include and including GROUP BY and HAVING clauses. In multiple-object views, you can use joins, subqueries, or combinations of the two to construct connections in the tables and views underlying the view.

In spite of the many kinds of views that can be created, there are always limits, varying from SQL to SQL. Basically, restrictions have two sources:

  • Elements not allowed in CREATE VIEW statements (ORDER BY and sometimes UNION). Check your system manuals for specifics.

  • Elements permitted in CREATE VIEW statements (computed columns, aggregates) that can limit the data modification permitted through the view because of the problem of interpreting data modification statements.

The view-updating problem is explained and illustrated later in this chapter. If you want users to be able to perform most functions through a view, you may decide to modify a perfectly legal CREATE VIEW statement to avoid these limitations.

Displaying Data Through Views

Creating a view doesn't produce a display. When SQL receives a CREATE VIEW command, it does not actually execute the SELECT statement that follows the keyword AS. Instead, it stores the SELECT statement in the system catalogs.

In order to see data through the view, query the view, just as you would a table.

SQL
select * 
from oaklanders

FirstName  LastName    Book
=========  ==========  ===============================================
Marjorie   Green       The Busy Executive's Database Guide
Stearns    MacFeather  Cooking with Computers: 
                       Surreptitious Balance Sheets
Marjorie   Green       You Can Combat Computer Stress!
Dick       Straight    Straight Talk About Computers
Livia      Karsen      Computer Phobic and Non-Phobic Individuals: 
                       Behavior Variations
Stearns    MacFeather  Computer Phobic and Non-Phobic Individuals: 
[6 rows]               Behavior Variations

A view in a SELECT statement works just like a table. You can

  • Change the order of the columns in the display;
  • Add display labels;
  • Limit the rows with WHERE conditions;
  • Group rows;
  • Limit group results with HAVING;
  • Sort the results.

For example, you could write a query of oaklanders like this, changing the position of columns, adding display labels, and sorting the results:

SQL
select Book as Title,  LastName as Surname
from oaklanders
order by Title
Title                                                      Surname
========================================================== ==========
Computer Phobic and Non-Phobic Individuals: 
Behavior Variations                                        Karsen
Computer Phobic and Non-Phobic Individuals: 
Behavior Variations                                        MacFeather
Cooking with Computers: Surreptitious Balance Sheets       MacFeather
Straight Talk About Computers                              Straight
The Busy Executive's Database Guide                        Green
You Can Combat Computer Stress!                            Green
[6 rows]

Here's a query of the oaklanders view with aggregates, GROUP BY, and HAVING:

SQL
select LastName, count(Book)
from oaklanders
group by LastName
having count(Book) > 1
LastName                                 count(titles.title)
======================================== ====================
Green                                    2
MacFeather                               2
[2 rows]

Dropping Views

The command for removing views is:

SYNTAX

DROP VIEW view_name

If a view depends on a table (or on another view) that has been dropped, you won't be able to use the view. However, if you create a new table (or view) with the same name to replace the dropped one, you may be able to use the view again, as long as the columns referenced in the view definition still exist. Check your system's reference manuals for details.

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