Oracle PL/SQL by Example, Fifth Edition (2015)

Many Oracle applications are built using multiple tiers, also known as N-tier architecture, where each tier represents a separate logical process. For example, a three-tier architecture would consist of three tiers: a data management tier, an application processing tier, and a presentation tier. In this architecture, the Oracle database resides in the data management tier, and the programs that make requests against this database reside in either the presentation tier or the application processing tier. Such programs can be written in many programming languages, including PL/SQL. An example of a three-tier architecture is shown in Figure 1.1.

Figure 1.1 Three-Tier Architecture

PL/SQL Architecture

While PL/SQL is just like any other programming language, its main distinction is that it is not a stand-alone programming language. Rather, PL/SQL is a part of the Oracle RDBMS as well as various Oracle development tools such as Oracle Application Express (APEX) and Oracle Forms and Reports. As a result, PL/SQL may reside in any layer of the multitier architecture.

No matter which layer PL/SQL resides in, any PL/SQL block or subroutine is processed by the PL/SQL engine, which is a special component of various Oracle products. As a result, it is very easy to move PL/SQL modules between various tiers. The PL/SQL engine processes and executes any PL/SQL statements and sends any SQL statements to the SQL statement processor. The SQL statement processor is always located on the Oracle server. Figure 1.2 illustrates the PL/SQL engine residing on the Oracle server.

Figure 1.2 The PL/SQL Engine and Oracle Server

When the PL/SQL engine is located on the server, the whole PL/SQL block is passed to the PL/SQL engine on the Oracle server. The PL/SQL engine processes the block according to the scheme depicted in Figure 1.2.

When the PL/SQL engine is located on the client, as it is in Oracle development tools, the PL/SQL processing is done on the client side. All SQL statements that are embedded within the PL/SQL block are sent to the Oracle server for further processing. When PL/SQL block contains no SQL statements, the entire block is executed on the client side.

Using PL/SQL has several advantages. For example, when you issue a SELECT statement in SQL*Plus or SQL Developer against the STUDENT table, it retrieves a list of students. The SELECT statement you issued at the client computer is sent to the database server to be executed. The results of this execution are then returned to the client. In turn, rows are displayed on your client machine.

Now, assume that you need to issue multiple SELECT statements. Each SELECT statement is a request against the database and is sent to the Oracle server. The results of each SELECT statement are sent back to the client. Each time a SELECT statement is executed, network traffic is generated. Hence, multiple SELECT statements will result in multiple round-trip transmissions, adding significantly to the network traffic.

When these SELECT statements are combined into a PL/SQL program, they are sent to the server as a single unit. The SELECT statements in this PL/SQL program are executed at the server. The server sends the results of these SELECT statements back to the client, also as a single unit. Therefore, a PL/SQL program encompassing multiple SELECT statements can be executed at the server and have all of the results returned to the client in the same round trip. This is obviously a more efficient process than having each SELECT statement execute independently. This model is illustrated in Figure 1.3.

Figure 1.3 PL/SQL in Client–Server Architecture

Figure 1.3 compares two applications. The first application uses four independent SQL statements that generate eight trips on the network. The second application combines SQL statements into a single PL/SQL block, which is then sent to the PL/SQL engine. The engine sends SQL statements to the SQL statement processor and checks the syntax of the PL/SQL statements. As you can see, only two trips are generated on the network with the second application.

In addition, applications written in PL/SQL are portable. They can run in any environment that Oracle products can run in. Because PL/SQL does not change from one environment to the next, different tools can use a PL/SQL script.

PL/SQL Block Structure

A block is the most basic unit in PL/SQL. All PL/SQL programs are combined into blocks. These blocks can also be nested within one another. Usually, PL/SQL blocks combine statements that represent a single logical task. Therefore, different tasks within a single program can be separated into blocks. With this structure, it is easier to understand and maintain the logic of the program.

PL/SQL blocks can be divided into two groups: named and anonymous. Named PL/SQL blocks are used when creating subroutines. These subroutines, which include procedures, functions, and packages, can be stored in the database and referenced by their names later. In addition, subroutines such as procedures and functions can be defined within the anonymous PL/SQL block. These subroutines exist as long as the block is executing and cannot be referenced outside the block. In other words, subroutines defined in one PL/SQL block cannot be called by another PL/SQL block or referenced by their names later. Subroutines are discussed in Chapters 19 through 21. Anonymous PL/SQL blocks, as you have probably guessed, do not have names. As a result, they cannot be stored in the database or referenced later.

PL/SQL blocks contain three sections: a declaration section, an executable section, and an exception-handling section. The executable section is the only mandatory section of the block; both the declaration and exception-handling sections are optional. As a result, a PL/SQL block has the structure illustrated in Listing 1.1.

Listing 1.1 PL/SQL Block Structure

DECLARE
Declaration statements
BEGIN
Executable statements
EXCEPTION
Exception-handling statements
END;

Declaration Section

The declaration section is the first section of the PL/SQL block. It contains definitions of PL/SQL identifiers such as variables, constants, cursors, and so on. PL/SQL identifiers are covered in detail throughout this book.

For Example

DECLARE
v_first_name VARCHAR2(35);
v_last_name VARCHAR2(35);

This example shows the declaration section of an anonymous PL/SQL block. It begins with the keyword DECLARE and contains two variable declarations. The names of the variables, v_first_name and v_last_name, are followed by their data types and sizes. Notice that a semicolon terminates each declaration.

Executable Section

The executable section is the next section of the PL/SQL block. It contains executable statements that allow you to manipulate the variables that have been declared in the declaration section.

For Example

BEGIN
SELECT first_name, last_name
INTO v_first_name, v_last_name
FROM student
WHERE student_id = 123;

DBMS_OUTPUT.PUT_LINE ('Student name: '||v_first_name||' '||v_last_name);
END;

This example shows the executable section of the PL/SQL block. It begins with the keyword BEGIN and contains a SELECT INTO statement from the STUDENT table. The first and last names for student ID 123 are selected into two variables: v_first_name and v_last_name.Chapter 3 contains a detailed explanation of the SELECT INTO statement. Next, the values of the variables, v_first_name and v_last_name, are displayed on the screen with the help of the DBMS_OUTPUT.PUT_LINE statement. This statement will be covered later in this chapter in greater detail. The end of the executable section of this block is marked by the keyword END.