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Advantages of Views

To clarify the advantages of using views, consider several possible users of the bookbiz database. Let's say that the promotion manager needs to know which authors are connected to which books and who has first, second, and third billing on the cover. Prices, sales, advances, royalties, and personal addresses are not of interest, but the promotion manager does need some information from each of the three tables: titles, authors, and titleauthors. Without a view, a query something like the following might be used:

SQL
select titles.title_id, au_ord, au_lname, au_fname
from authors, titles, titleauthors
where authors.au_id = titleauthors.au_id and
   titles.title_id = titleauthors.title_id

This query involves a lot of typing, and there are any number of places where an error might slip in. Quite a bit of knowledge of the database is required, too. Creating a view called books that is based on this SELECT statement would facilitate the use of this particular set of data. Here's the statement that creates the view (unlike the oaklanders view, this one inherits column names from the SELECT clause):

SQL
create view books
as
select titles.title_id, au_ord, au_lname, au_fname
from authors, titles, titleauthors
where authors.au_id = titleauthors.au_id and
   titles.title_id = titleauthors.title_id

Now the promotion manager can use the view to get the same results without thinking about joins or select lists or search conditions:

SQL
select * 
from books
title_id au_ord au_lname                      au_fname
======== ====== ============================= ==========
PC8888        1 Dull                          Ann
PC8888        2 Hunter                        Sheryl
BU1032        1 Bennet                        Abraham
BU1032        2 Green                         Marjorie
PS7777        1 Locksley                      Chastity
PS3333        1 White                         Johnson
BU1111        1 MacFeather                    Stearns
BU1111        2 O'Leary                       Michael
MC2222        1 del Castillo                  Innes
TC7777        1 Yokomoto                      Akiko
TC7777        2 O'Leary                       Michael
TC7777        3 Gringlesby                    Burt
TC4203        1 Blotchet-Halls                Reginald
PC1035        1 Carson                        Cheryl
BU2075        1 Green                         Marjorie
PS2091        1 Ringer                        Albert
PS2091        2 Ringer                        Anne
PS2106        1 Ringer                        Albert
MC3021        1 DeFrance                      Michel
MC3021        2 Ringer                        Anne
TC3218        1 Panteley                      Sylvia
BU7832        1 Straight                      Dick
PS1372        1 Karsen                        Livia
PS1372        2 MacFeather                    Stearns
PC9999        1 Locksley                      Chastity
[25 rows]

An accountant might want to create a different view. Author order on the title page doesn't matter, just the bottom line: To whom should checks be written and for how much. The query involves computing how many books were sold at what price, with what percentage rate for each author:

SQL
select au_lname, au_fname,
 sum(price*ytd_sales*royalty*royaltyshare) as Total_Income
from authors, titles, titleauthors, roysched
where authors.au_id = titleauthors.au_id
 and titles.title_id = titleauthors.title_id
 and titles.title_id = roysched.title_id
 and ytd_sales between lorange and hirange
group by au_lname, au_fname

If the accountant uses this SELECT statement to create a view named royaltychecks, the equivalent query is this:

SQL
select * 
from royaltychecks

The results (who gets a check and for how much) are as follows:

Results
au_lname           au_fname              Total_Income
================== ===================== ===============
Bennet             Abraham                       7368.54
Blotchet-Halls     Reginald                     46390.01
Carson             Cheryl                       60336.16
DeFrance           Michel                       43346.33
del Castillo       Innes                         7312.76
Dull               Ann                           8190.00
Green              Marjorie                     48688.14
Gringlesby         Burt                          3684.27
Hunter             Sheryl                        8190.00
Karsen             Livia                         1169.72
Locksley           Chastity                      6001.46
MacFeather         Stearns                       5494.60
O'Leary            Michael                       7087.40
Panteley           Sylvia                        1535.63
Ringer             Albert                        2881.97
Ringer             Anne                         17142.04
Straight           Dick                         12280.91
White              Johnson                      12211.93
Yokomoto           Akiko                         4912.36
(19 rows affected)

Finally, consider an executive at the parent publishing company who needs to find out how the different categories of books are doing at each subsidiary. The executive can use a query something like this:

SQL
select pub_id, type, sum(price*ytd_sales),
  avg(price), avg(ytd_sales)
from titles
group by pub_id, type

However, the executive may not want to bother with anything so complex. A much simpler query is

SQL
select * 
from currentinfo
PUB# TYPE         INCOME     AVG_PRICE  AVG_SALES
==== ============ ========== ========== ==========
1389 popular_comp   40901.00      41.48    6437.50
1389 business      330696.30      27.31    4022.00
0736 psychology    244504.92      25.70    1987.80
0877 mod_cook      349915.22      21.49   12139.00
0877 trad_cook     469522.50      30.96    6522.00
0736 business      243198.78      12.99   18722.00
0877 (NULL)           (NULL)     (NULL)     (NULL)
[7 rows]

Using this view, the busy executive can quickly see which publishing lines are making money, and he or she can compare the relationship among income, average price, and average sales.

SQL Variants

You might see some formatting differences in results, depending on what datatypes your systems uses and how your system displays numbers. Microsoft SQL Server, for example, shows AVG_SALES values as whole numbers:

SQL Server
PUB#  TYPE          INCOME           AVG_PRICE     AVG_SALES
----  ------------  ---------------  ------------  ---------
0877  NULL                     NULL          NULL       NULL
0736  business          243198.7800       12.9900      18722
1389  business          330696.3000       27.3100       4022
0877  mod_cook          349915.2200       21.4900      12139
1389  popular_comp      540901.0000       41.4750       6437
0736  psychology        244504.9200       25.7040       1987
0877  trad_cook         469522.5000       30.9633       6522
(7 row(s) affected) 

As the previous examples demonstrate, you can use views to focus, simplify, and customize each user's perception of the database. In addition, views provide a security mechanism. Finally, they can protect users from the effects of changes in the database structure, providing independence.

Focus, Simplification, and Customization

Views allow the promotion manager, the accountant, and the executive in the previous examples to focus in on the particular data and tasks. No extraneous or distracting information gets in the way.

Working with the data is simpler, too. When favorite joins, projections, and/or selections are already defined as views, it's relatively simple to add other clauses. Constructing the entire underlying query, on the other hand, could be a daunting prospect.

Views are a good way of customizing a database or tailoring it to suit a variety of users with dissimilar interests and skill levels. Our three users see the data in different ways, even when they're looking at the same three tables at the same time.

Security

Views provide security by hiding sensitive or irrelevant parts of the database. If permissions are set up properly, the accountant can find out how big an author's check should be, but can't look at the underlying figures or compare his or her own paycheck to a coworker's. You can restrict the accountant's access in the database to just those views that are relevant to accounting. Using views as a security mechanism is discussed in more detail in Chapter 10.

Independence

Finally, there's the issue of independence. From time to time, you may have to modify the structure of the database, but there's no reason that users should suffer from these changes. For example, say you split the titles table into two new tables and drop titles. The new tables are shown in Figure 9.1.

Figure 9.1Figure 9.1 Splitting a Table into Two Tables

Notice that the old titles table can be regenerated by joining the title_id columns of the two new tables. To shield the changed structure of the database from users, you can create a view that is the join of the two new tables (Figure 9.2). You can even name it titles (although here it's called titlesview). In most systems, doing so means you won't have to change any views that were based wholly or in part on the old titles.

Figure 9.2Figure 9.2 Uniting Two Base Tables in a Single View

Restrictions on updating data through views (explained later in this chapter) limit the independence of this kind of view. Certain data modification statements on the new titles may not be allowed.

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