Understanding the Main Features of PL/SQL

implementation details, so that you can change the details without affecting client programs.

In addition, object types allow for realistic data modeling. Complex real-world entities and relationships map directly into object types. This direct mapping helps your programs better reflect the world they are trying to simulate.

Understanding the Main Features of PL/SQL

PL/SQL combines the data-manipulating power of SQL with the processing power of procedural languages.

You can control program flow with statements like IF and LOOP. As with other procedural programming languages, you can declare variables, define procedures and functions, and trap runtime errors.

PL/SQL lets you break complex problems down into easily understandable procedural code, and reuse this code across multiple applications. When a problem can be solved through plain SQL, you can issue SQL commands directly inside your PL/SQL programs, without learning new APIs. PL/SQL's data types correspond with SQL's column types, making it easy to interchange PL/SQL variables with data inside a table.

Block Structure

The basic units (procedures, functions, and anonymous blocks) that make up a PL/SQL program are logical blocks, which can be nested inside one another.

A block groups related declarations and statements. You can place declarations close to where they are used, even inside a large subprogram. The declarations are local to the block and cease to exist when the block completes, helping to avoid cluttered namespaces for variables and procedures.

As Figure 1–2 shows, a PL/SQL block has three parts: a declarative part, an executable part, and an exception-handling part that deals with error conditions. Only the executable part is required.

First comes the declarative part, where you define types, variables, and similar items. These items are manipulated in the executable part. Exceptions raised during execution can be dealt with in the exception-handling part.

Figure 1–2 Block Structure

[DECLARE

--declarations] BEGIN

--statements [EXCEPTION

--handlers]

END;

You can nest blocks in the executable and exception-handling parts of a PL/SQL block or subprogram but not in the declarative part. You can define local subprograms in the declarative part of any block. You can call local subprograms only from the block in which they are defined.

1-4 PL/SQL User's Guide and Reference

Understanding the Main Features of PL/SQL

Variables and Constants

PL/SQL lets you declare constants and variables, then use them in SQL and procedural statements anywhere an expression can be used. You must declare a constant or variable before referencing it in any other statements.

Declaring Variables

Variables can have any SQL datatype, such as CHAR, DATE, or NUMBER, or a PL/SQL-only datatype, such as BOOLEAN or PLS_INTEGER. For example, assume that you want to declare a variable named part_no to hold 4-digit numbers and a variable named in_stock to hold the Boolean value TRUE or FALSE. You declare these variables as follows:

part_no NUMBER(4); in_stock BOOLEAN;

You can also declare nested tables, variable-size arrays (varrays for short), and records using the TABLE, VARRAY, and RECORD composite datatypes.

Assigning Values to a Variable

You can assign values to a variable in three ways. The first way uses the assignment operator (:=), a colon followed by an equal sign. You place the variable to the left of the operator and an expression (which can include function calls) to the right. A few examples follow:

tax := price * tax_rate; valid_id := FALSE;

bonus := current_salary * 0.10;

wages := gross_pay(emp_id, st_hrs, ot_hrs) - deductions;

The second way to assign values to a variable is by selecting (or fetching) database values into it. In the example below, you have Oracle compute a 10% bonus when you select the salary of an employee. Now, you can use the variable bonus in another computation or insert its value into a database table.

SELECT salary * 0.10 INTO bonus FROM employees WHERE employee_id = emp_id;

The third way to assign values to a variable is by passing it as an OUT or IN OUT parameter to a subprogram, and doing the assignment inside the subprogram. The following example passes a variable to a subprogram, and the subprogram updates the variable:

DECLARE

my_sal REAL(7,2);

PROCEDURE adjust_salary (emp_id INT, salary IN OUT REAL) IS ...

BEGIN

SELECT AVG(sal) INTO my_sal FROM emp;

adjust_salary(7788, my_sal); -- assigns a new value to my_sal

Declaring Constants

Declaring a constant is like declaring a variable except that you must add the keyword CONSTANT and immediately assign a value to the constant. No further assignments to the constant are allowed. The following example declares a constant:

credit_limit CONSTANT NUMBER := 5000.00;

Overview of PL/SQL 1-5

Understanding the Main Features of PL/SQL

Processing Queries with PL/SQL

Processing a SQL query with PL/SQL is like processing files with other languages. For example, a Perl program opens a file, reads the file contents, processes each line, then closes the file. In the same way, a PL/SQL program issues a query and processes the rows from the result set:

FOR someone IN (SELECT * FROM employees)

LOOP

DBMS_OUTPUT.PUT_LINE('First name = ' || someone.first_name);

DBMS_OUTPUT.PUT_LINE('Last name = ' || someone.last_name);

END LOOP;

You can use a simple loop like the one shown here, or you can control the process precisely by using individual statements to perform the query, retrieve data, and finish processing.

Declaring PL/SQL Variables

As part of the declaration for each PL/SQL variable, you declare its datatype. Usually, this datatype is one of the types shared between PL/SQL and SQL, such as NUMBER or VARCHAR2(length). For easier maintenance of code that interacts with the database, you can also use the special qualifiers %TYPE and %ROWTYPE to declare variables that hold table columns or table rows.

%TYPE

The %TYPE attribute provides the datatype of a variable or database column. This is particularly useful when declaring variables that will hold database values. For example, assume there is a column named title in a table named books. To declare a variable named my_title that has the same datatype as column title, use dot notation and the %TYPE attribute, as follows:

my_title books.title%TYPE;

Declaring my_title with %TYPE has two advantages. First, you need not know the exact datatype of title. Second, if you change the database definition of title (make it a longer character string for example), the datatype of my_title changes accordingly at run time.

%ROWTYPE

In PL/SQL, records are used to group data. A record consists of a number of related fields in which data values can be stored. The %ROWTYPE attribute provides a record type that represents a row in a table. The record can store an entire row of data selected from the table or fetched from a cursor or cursor variable.

Columns in a row and corresponding fields in a record have the same names and datatypes. In the example below, you declare a record named dept_rec. Its fields have the same names and datatypes as the columns in the dept table.

DECLARE

dept_rec dept%ROWTYPE; -- declare record variable

You use dot notation to reference fields, as the following example shows:

my_deptno := dept_rec.deptno;

1-6 PL/SQL User's Guide and Reference

Understanding the Main Features of PL/SQL

If you declare a cursor that retrieves the last name, salary, hire date, and job title of an employee, you can use %ROWTYPE to declare a record that stores the same information, as follows:

DECLARE

CURSOR c1 IS

SELECT ename, sal, hiredate, job FROM emp;

emp_rec c1%ROWTYPE; -- declare record variable that represents -- a row fetched from the emp table

When you execute the statement

FETCH c1 INTO emp_rec;

the value in the ename column of the emp table is assigned to the ename field of emp_rec, the value in the sal column is assigned to the sal field, and so on.

Control Structures

Control structures are the most important PL/SQL extension to SQL. Not only does PL/SQL let you manipulate Oracle data, it lets you process the data using conditional, iterative, and sequential flow-of-control statements such as IF-THEN-ELSE, CASE,

FOR-LOOP, WHILE-LOOP, EXIT-WHEN, and GOTO.

Conditional Control

Often, it is necessary to take alternative actions depending on circumstances. The IF-THEN-ELSE statement lets you execute a sequence of statements conditionally. The IF clause checks a condition; the THEN clause defines what to do if the condition is true; the ELSE clause defines what to do if the condition is false or null.

Consider the program below, which processes a bank transaction. Before allowing you to withdraw $500 from account 3, it makes sure the account has sufficient funds to cover the withdrawal. If the funds are available, the program debits the account. Otherwise, the program inserts a record into an audit table.

-- available online in file 'examp2' DECLARE

acct_balance NUMBER(11,2);

SELECT bal INTO acct_balance FROM accounts WHERE account_id = acct

FOR UPDATE OF bal;

IF acct_balance >= debit_amt THEN

UPDATE accounts SET bal = bal - debit_amt WHERE account_id = acct;

ELSE

INSERT INTO temp VALUES

(acct, acct_balance, 'Insufficient funds');

-- insert account, current balance, and message

END IF;

COMMIT;

END;

To choose among several values or courses of action, you can use CASE constructs. The CASE expression evaluates a condition and returns a value for each case. The case statement evaluates a condition and performs an action (which might be an entire PL/SQL block) for each case.

Overview of PL/SQL 1-7

Understanding the Main Features of PL/SQL

--This CASE statement performs different actions based

--on a set of conditional tests.

CASE

WHEN shape = 'square' THEN area := side * side; WHEN shape = 'circle' THEN

BEGIN

area := pi * (radius * radius);

DBMS_OUTPUT.PUT_LINE('Value is not exact because pi is irrational.'); END;

WHEN shape = 'rectangle' THEN area := length * width; ELSE

BEGIN

DBMS_OUTPUT.PUT_LINE('No formula to calculate area of a' || shape); RAISE PROGRAM_ERROR;

END; END CASE;

A sequence of statements that uses query results to select alternative actions is common in database applications. Another common sequence inserts or deletes a row only if an associated entry is found in another table. You can bundle these common sequences into a PL/SQL block using conditional logic.

Iterative Control

LOOP statements let you execute a sequence of statements multiple times. You place the keyword LOOP before the first statement in the sequence and the keywords END LOOP after the last statement in the sequence. The following example shows the simplest kind of loop, which repeats a sequence of statements continually:

LOOP

-- sequence of statements END LOOP;

The FOR-LOOP statement lets you specify a range of integers, then execute a sequence of statements once for each integer in the range. For example, the following loop inserts 500 numbers and their square roots into a database table:

FOR num IN 1..500 LOOP

INSERT INTO roots VALUES (num, SQRT(num));

END LOOP;

The WHILE-LOOP statement associates a condition with a sequence of statements. Before each iteration of the loop, the condition is evaluated. If the condition is true, the sequence of statements is executed, then control resumes at the top of the loop. If the condition is false or null, the loop is bypassed and control passes to the next statement.

In the following example, you find the first employee who has a salary over $2500 and is higher in the chain of command than employee 7499:

-- available online in file 'examp3'

starting_empno emp.empno%TYPE := 7499; BEGIN

SELECT mgr INTO mgr_num FROM emp WHERE empno = starting_empno;

WHILE salary <= 2500 LOOP

SELECT sal, mgr, ename INTO salary, mgr_num, last_name FROM emp WHERE empno = mgr_num;

1-8 PL/SQL User's Guide and Reference