Congratulations on your decision to read this book, Sams Teach Yourself PL/SQL in 21 Days, Second Edition!
If you are new to the Oracle
environment, this book will help you learn and master Oracle's built-in
procedural language quickly. Knowledge of PL/SQL (Procedural Language/Structured
Query Language) is becoming a fundamental necessity no matter which of
Oracle's many products you use.
Today, on your first day, you will accomplish these tasks:
Learn what PL/SQL is and why you should master it
Learn how PL/SQL relates to other Oracle products
Learn what resources you need to finish this book
Write your first PL/SQL function
Over the remaining 20 days, you'll delve deeper into the power and
capabilities of this language and learn how to leverage its power in your
applications regardless of whether you are doing client/server programming with
Oracle's tools (such as Developer/2000), using other front-end tools (such
as PowerBuilder), or simply writing some batch jobs that run on the server.
What Is PL/SQL?
PL/SQL is a procedural language that Oracle developed as an extension to
standard SQL to provide a way to execute procedural logic on the database.
New Term - If you have worked with relational
databases in the past, you are no doubt familiar with SQL, which stands for
Structured Query Language. SQL itself is a powerful declarative language.
It is declarative in the sense that you describe the results that you
want but not how they are obtained. This is good because you can insulate an
application from the specifics of how the data is physically stored. A competent
SQL programmer can also push a great deal of processing work back to the server
level through the creative use of SQL.
There are limits, though, to what you can accomplish with a single
declarative query. The real world is seldom as neat and clean as we would like
it to be. Developers often find themselves needing to execute several queries in
succession and process the specific results of one query before going on to the
next. This leads to two problems in a client/server environment:
The procedural logic, that is, the definition of the process, resides on
client machines.
The need to look at the data from one query and use it as the basis for
the next query results in an increased amount of network traffic.
Why are these problems? The procedural logic on client machines can quickly
become out of sync if the software is upgraded. It can also be implemented
incorrectly, resulting in a loss of database integrity. The need to pull down
large amounts of intermediate data to a client results in a long wait for the
end users who must sit there staring at the hourglass while the data is
transferred to their machines. The cumulative effects of a number of clients
pulling large amounts of data across the network further decrease
performance.
PL/SQL provides a mechanism for developers to add a procedural component at
the server level. It has been enhanced to the point where developers now have
access to all the features of a full-featured procedural language at the server
level. It also forms the basis for programming in Oracle's continually
evolving set of client/server development tools, most notably
Developer/2000.
Why Learn PL/SQL?
If you are developing with Oracle products, Developer/2000 for example, the
answer to this question is simple. You need to know PL/SQL because those
products use PL/SQL for any procedural code. But what if you don't develop
with Oracle's products? What if all you use is Oracle's database
engine? Is PL/SQL of any use to you? Yes! Absolutely it is.
Regardless of the front-end tool that you are using, you can use PL/SQL to
perform processing on the server rather than the client. You can use PL/SQL to
encapsulate business rules and other complicated logic. It provides for
modularity and abstraction. You can use it in database triggers to code complex
constraints, which enforce database integrity; to log changes; and to replicate
data. PL/SQL can also be used with stored procedures and functions to provide
enhanced database security. Finally, it provides you with a level of platform
independence. Oracle is implemented on many hardware platforms, but PL/SQL is
the same on all of them. It makes no difference whether you are running Personal
Oracle on a laptop or Oracle8i Enterprise on UNIX.
Regardless of what development tools you use, if you are developing in an
Oracle environment, your knowledge of PL/SQL and your ability to apply it will
give you a competitive advantage against those who do not have that knowledge.
With PL/SQL you have the power to make your applications more robust, more
efficient, and more secure.
SQL, SQL*Plus, PL/SQL: What's the Difference?
This question has bedeviled many people new to Oracle. There are several
products with the letters "SQL" in the title, and these three,
SQL*Plus, SQL, and PL/SQL, are often used together. Because of this, it's
easy to become confused as to which product is doing the work and where the work
is being done. This section briefly describes each of these three products.
SQL
SQL stands for Structured Query Language. This has become the lingua
franca of database access languages. It has been adopted by the
International Standards Organization (ISO) and has also been adopted by the
American National Standards Institute (ANSI). When you code statements such as
SELECT, INSERT, UPDATE, and DELETE, SQL is
the language you are using. It is a declarative language and is always executed
on the database server. Often you will find yourself coding SQL statements in a
development tool, such as PowerBuilder or Visual Basic, but at runtime those
statements are sent to the server for execution.
PL/SQL
PL/SQL is Oracle's Procedural Language extension to SQL. It, too,
usually runs on the database server, but some Oracle products such as
Developer/2000 also contain a PL/SQL engine that resides on the client. Thus,
you can run your PL/SQL code on either the client or the server depending on
which is more appropriate for the task at hand. Unlike SQL, PL/SQL is
procedural, not declarative. This means that your code specifies exactly
how things get done. As in SQL, however, you need some way to send your PL/SQL
code up to the server for execution. PL/SQL also enables you to embed SQL
statements within its procedural code. This tight-knit relationship between
PL/SQL, SQL, and SQL*Plus is the cause for some of the confusion between the
products.
SQL*Plus
SQL*Plus is an interactive program that allows you to type in and execute SQL
statements. It also enables you to type in PL/SQL code and send it to the server
to be executed. SQL*Plus is one of the most common front ends used to develop
and create stored PL/SQL procedures and functions.
What happens when you run SQL*Plus and type in a SQL statement? Where does
the processing take place? What exactly does SQL*Plus do, and what does the
database do? If you are in a Windows environment and you have a database server
somewhere on the network, the following things happen:
SQL*Plus transmits your SQL query over the network to the database
server.
SQL*Plus waits for a reply from the database server.
The database server executes the query and transmits the results back to
SQL*Plus.
SQL*Plus displays the query results on your computer screen.
Even if you're not running in a networked Windows environment, the same
things happen. The only difference might be that the database server and
SQL*Plus are running on the same physical machine. This would be true, for
example, if you were running Personal Oracle on a single PC.
PL/SQL is executed in much the same manner. Type a PL/SQL block into
SQL*Plus, and it is transmitted to the database server for execution. If there
are any SQL statements in the PL/SQL code, they are sent to the server's
SQL engine for execution, and the results are returned back to the PL/SQL
program.
The important thing is that SQL*Plus does not execute your SQL queries. SQL*Plus
also does not execute your PL/SQL code. SQL*Plus simply serves as your window
into the Oracle database, which is where the real action takes place. Figure
1.1 illustrates this relationship.
Relationship of SQL*Plus, PL/SQL, and Oracle.
Several other tools besides SQL*Plus can serve as your window to the
database. Server Manager, which has an interface similar to SQL*Plus, is
one such tool, although Oracle plans to stop supporting it sometime in the
future. If you have Oracle Enterprise Manager installed, you should take a look
at SQLPlus Worksheet. SQLPlus Worksheet is a GUI tool that is fully compatible
with SQL*Plus but is much easier to use. If you are a Developer 2000 programmer,
you'll have access to Oracle's Procedure Builder--a tool designed
for developing and debugging PL/SQL code. You'll read more about SQLPlus
Worksheet and Procedure Builder later in this chapter.
SQL*Plus is used for most of the examples in this book because of its
universal availability to developers. It is perhaps still the most widely used
tool to develop, test, and create PL/SQL stored subprograms and SQL queries.
Note - In addition to Oracle's tools, several third-party vendors
also have tools that can be used to develop PL/SQL code. Some of the major
products in this space are
What You Need to Finish This Book
In order to try the examples and complete the exercises in this book, you
will need access to
Note - Where possible, the exercises and
examples in this book have been designed to run equally well under both Oracle8
and Oracle8i. Many, especially those in the first nine days, will even run under
Oracle7. However, Oracle8i contains many new features that are not available in
previous releases. Days 10, 11, 12, 20, and 21, in particular, are heavily
focused on the new 8i features.
If you do not currently have access to an Oracle database, there are at least
two ways to get your hands on one. For a nominal cost, you can visit
Oracle's online store and purchase a 30-day evaluation version of almost
any Oracle product, including the database. You can get to the online Oracle
Store from Oracle's home page,
http://www.oracle.com.
Another option is to join the Oracle Technology Network (OTN). OTN members can
download developer-licensed copies of Oracle's database software at no
charge. OTN members also have the option of subscribing to various technology
tracks in order to get regular shipments of Oracle software CDs. You can
register as an OTN member at no cost. The URL to visit is
http://technet.oracle.com.
You will need these database privileges roles:
CREATE PROCEDURE
CREATE SEQUENCE
CREATE SESSION
CREATE TABLE
CREATE TRIGGER
CREATE VIEW
CREATE TYPE
The following Oracle-supplied packages should be available:
DBMS_OUTPUT
DBMS_SQL
UTL_FILE
DBMS_PIPE
DBMS_ALERT
Your database administrator can help you verify that these packages are
available to you.
If you are using Oracle8i Personal Edition, you can verify the existence of
these packages by logging on as the user SYSTEM and issuing the following
query:
SELECT object_name
FROM dba_objects
WHERE owner='SYS'
AND object_type = 'PACKAGE';
The resulting list will show you all packages in the database owned by the
user SYS. The packages named in this chapter should be in that list. Of those,
the DBMS_OUTPUT is the most essential and is used throughout most of
the exercises and examples to display results. The other packages are discussed
only in specific chapters.
Caution - I recommend that you do not use a
production database and that you create the sample tables in a schema that is
not shared with other users. If you are using Personal Oracle on your own PC,
you won't have a problem with this. If you are using an employer's
facilities, you might want to discuss use of the database with your
employer's database administrator, or DBA, as they are often called. There
is nothing inherently dangerous in any of the exercises or examples, but there
is always the risk that a coding mistake, such as an infinite loop, might tie up
CPU or I/O resources. It's always good etiquette to minimize the potential
impact of your mistakes on other developers and end users.
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