PL/SQL can help you reduce overhead, improve performance, and increase productivity. For example, without PL/SQL, Oracle must process SQL statements one at a time. Each SQL statement results in another call to the Server and consequently, a higher overhead. However, with PL/SQL, you can send an entire block of SQL statements to the Server. This minimizes communication between your application and Oracle.

PL/SQL is tightly integrated with the Oracle Server. For example, most PL/SQL datatypes are native to the Oracle data dictionary. Furthermore, you can use the %TYPE attribute to base variable declarations on column definitions stored in the data dictionary, as the following example shows:

job_title emp.job%TYPE;

That way, you need not know the exact datatype of the column. Furthermore, if a column definition changes, the variable declaration changes accordingly and automatically. This provides data independence, reduces maintenance costs, and allows programs to adapt as the database changes.

With PL/SQL, you need not use the DECLARE, OPEN, FETCH, and CLOSE statements to define and manipulate a cursor. Instead, you can use a cursor FOR loop, which implicitly declares its loop index as a record, opens the cursor associated with a given query, repeatedly fetches data from the cursor into the record, then closes the cursor. An example follows:

DECLARE
 ...
BEGIN
 FOR emprec IN (SELECT empno, sal, comm FROM emp) LOOP
 IF emprec.comm / emprec.sal > 0.25 THEN ...
 ...
 END LOOP;
END;

Notice that you use dot notation to reference fields in the record.

PL/SQL has two types of subprograms called procedures and functions, which aid application development by letting you isolate operations. Generally, you use a procedure to perform an action and a function to compute a value.

Procedures and functions provide extensibility. That is, they let you tailor the PL/SQL language to suit your needs. For example, if you need a procedure that creates a new department, just write your own as follows:

PROCEDURE create_dept
 (new_dname IN CHAR(14),
 new_loc IN CHAR(13),
 new_deptno OUT NUMBER(2)) IS
BEGIN
 SELECT deptno_seq.NEXTVAL INTO new_deptno FROM dual;
 INSERT INTO dept VALUES (new_deptno, new_dname, new_loc);
END create_dept;

When called, this procedure accepts a new department name and location, selects the next value in a department-number database sequence, inserts the new number, name, and location into the dept table, then returns the new number to the caller.

You can store subprograms in the database (using CREATE FUNCTION and CREATE PROCEDURE) that can be called from multiple applications without needing to be recompiled each time.

You use parameter modes to define the behavior of formal parameters. There are three parameter modes: IN (the default), OUT, and IN OUT. An 7IN parameter lets you pass values to the subprogram being called. An OUT parameter lets you return values to the caller of a subprogram. An IN OUT parameter lets you pass initial values to the subprogram being called and return updated values to the caller.

The datatype of each actual parameter must be convertible to the datatype of its corresponding formal parameter. Table 3-6 shows the legal conversions between datatypes.

PL/SQL lets you bundle logically related types, program objects, and subprograms into a package. Packages can be compiled and stored in an Oracle database, where their contents can be shared by multiple applications.

Packages usually have two parts: a specification and a body. The specification is the interface to your applications; it declares the types, constants, variables, exceptions, cursors, and subprograms available for use. The body defines cursors and subprograms and so implements the specification. In the following example, you "package" two employment procedures:

PACKAGE emp_actions IS -- package specification
 PROCEDURE hire_employee (empno NUMBER, ename CHAR, ...);
 PROCEDURE fire_employee (emp_id NUMBER);
END emp_actions;
PACKAGE BODY emp_actions IS -- package body
 PROCEDURE hire_employee (empno NUMBER, ename CHAR, ...) IS
 BEGIN
 INSERT INTO emp VALUES (empno, ename, ...);
 END hire_employee;
 PROCEDURE fire_employee (emp_id NUMBER) IS
 BEGIN
 DELETE FROM emp WHERE empno = emp_id;
 END fire_employee;
END emp_actions;

Only the declarations in the package specification are visible and accessible to applications. Implementation details in the package body are hidden and inaccessible.

PL/SQL provides a composite datatype named TABLE. Objects of type TABLE are called PL/SQL tables, which are modelled as (but not the same as) database tables. PL/SQL tables have only one column and use a primary key to give you array-like access to rows. The column can belong to any scalar type (such as CHAR, DATE, or NUMBER), but the primary key must belong to type BINARY_INTEGER.

You can declare PL/SQL table types in the declarative part of any block, procedure, function, or package. In the following example, you declare a TABLE type called NumTabTyp:

DECLARE
 TYPE NumTabTyp IS TABLE OF NUMBER
 INDEX BY BINARY_INTEGER;
 ...
BEGIN
 ...
END;

After you define type NumTabTyp, you can declare PL/SQL tables of that type, as the next example shows:

num_tab NumTabTyp;

The identifier num_tab represents an entire PL/SQL table.

You reference rows in a PL/SQL table using array-like syntax to specify the primary key value. For example, you reference the ninth row in the PL/SQL table named num_tab as follows:

num_tab(9) ...

You can use the %ROWTYPE attribute to declare a record that represents a row in a database table or a row fetched by a cursor. However, you cannot specify the datatypes of fields in the record or define fields of your own. The composite datatype RECORD lifts those restrictions.

Objects of type RECORD are called records. Unlike PL/SQL tables, records have uniquely named fields, which can belong to different datatypes. For example, suppose you have different kinds of data about an employee such as name, salary, hire date, and so on. This data is dissimilar in type but logically related. A record that contains such fields as the name, salary, and hire date of an employee would let you treat the data as a logical unit.

You can declare record types and objects in the declarative part of any block, procedure, function, or package. In the following example, you declare a RECORD type called DeptRecTyp:

DECLARE
 TYPE DeptRecTyp IS RECORD
 (deptno NUMBER(4) NOT NULL := 10, -- must initialize
 dname CHAR(9),
 loc CHAR(14));

Notice that the field declarations are like variable declarations. Each field has a unique name and specific datatype. You can add the NOT NULL option to any field declaration and so prevent the assigning of nulls to that field. However, you must initialize NOT NULL fields.

After you define type DeptRecTyp, you can declare records of that type, as the next example shows:

dept_rec DeptRecTyp;

The identifier dept_rec represents an entire record.

You use dot notation to reference individual fields in a record. For example, you reference the dname field in the dept_rec record as follows:

dept_rec.dname ...

The following example illustrates the use of host variables with PL/SQL. The program prompts the user for an employee number, then displays the job title, hire date, and salary of that employee.

EXEC SQL BEGIN DECLARE SECTION;
 username CHARACTER(20);
 password CHARACTER(20);
 emp_number INTEGER;
 job_title CHARACTER(20);
 hire_date CHARACTER(9);
 salary REAL;
EXEC SQL END DECLARE SECTION;
EXEC SQL INCLUDE SQLCA;
display 'Username? ';
read username;
display 'Password? ';
read password;
EXEC SQL WHENEVER SQLERROR DO sql_error;
EXEC SQL CONNECT :username IDENTIFIED BY :password;
display 'Connected to Oracle';
LOOP
 display 'Employee Number (0 to end)? ';
 read emp_number;
 IF emp_number = 0 THEN
 EXEC SQL COMMIT WORK RELEASE;
 display 'Exiting program';
 exit program;
 ENDIF;
 ---------------- begin PL/SQL block -----------------
 EXEC SQL EXECUTE
 BEGIN
 SELECT job, hiredate, sal
 INTO :job_title, :hire_date, :salary
 FROM emp
 WHERE empno = :emp_number;
 END;
 END-EXEC;
 ---------------- end PL/SQL block -----------------
 display 'Number Job Title Hire Date Salary';
 display '------------------------------------';
 display emp_number, job_title, hire_date, salary;
ENDLOOP;
...
ROUTINE sql_error
BEGIN
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 EXEC SQL ROLLBACK WORK RELEASE;
 display 'Processing error';
 exit program with an error;
END sql_error;

Notice that the host variable emp_number is set before the PL/SQL block is entered, and the host variables job_title, hire_date, and salary are set inside the block.

In the example, you prompt the user for a bank account number, transaction type, and transaction amount, then debit or credit the account. If the account does not exist, you raise an exception. When the transaction is complete, you display its status.

EXEC SQL BEGIN DECLARE SECTION;
 username CHARACTER(20);
 password CHARACTER(20);
 acct_num INTEGER;
 trans_type CHARACTER(1);
 trans_amt REAL;
 status CHARACTER(80);
EXEC SQL END DECLARE SECTION;
EXEC SQL INCLUDE SQLCA;
display 'Username? ';
read username;
display 'Password? ';
read password;
EXEC SQL WHENEVER SQLERROR DO sql_error;
EXEC SQL CONNECT :username IDENTIFIED BY :password;
display 'Connected to Oracle';
LOOP
 display 'Account Number (0 to end)? ';
 read acct_num;
 IF acct_num = 0 THEN
 EXEC SQL COMMIT WORK RELEASE;
 display 'Exiting program';
 exit program;
 ENDIF;
 display 'Transaction Type - D)ebit or C)redit? '
 read trans_type;
 display 'Transaction Amount? '
 read trans_amt;
 --------------------- begin PL/SQL block -------------------
 EXEC SQL EXECUTE
 DECLARE
 old_bal NUMBER(9,2);
 err_msg CHAR(70);
 nonexistent EXCEPTION;
 BEGIN
 :trans_type := UPPER(:trans_type);
 IF :trans_type = 'C' THEN -- credit the account
 UPDATE accts SET bal = bal + :trans_amt
 WHERE acctid = :acct_num;
 IF SQL%ROWCOUNT = 0 THEN -- no rows affected
 RAISE nonexistent;
 ELSE
 :status := 'Credit applied';
 END IF;
 ELSIF :trans_type = 'D' THEN -- debit the account
 SELECT bal INTO old_bal FROM accts
 WHERE acctid = :acct_num;
 IF old_bal >= :trans_amt THEN -- enough funds
 UPDATE accts SET bal = bal - :trans_amt
 WHERE acctid = :acct_num;
 :status := 'Debit applied';
 ELSE
 :status := 'Insufficient funds';
 END IF;
 ELSE
 :status := 'Invalid type: ' || :trans_type;
 END IF;
 COMMIT;
 EXCEPTION
 WHEN NO_DATA_FOUND OR nonexistent THEN
 :status := 'Nonexistent account';
 WHEN OTHERS THEN
 err_msg := SUBSTR(SQLERRM, 1, 70);
 :status := 'Error: ' || err_msg;
 END;
 END-EXEC;
 ------------------- end PL/SQL block -----------------------
 display 'Status: ', status;
ENDLOOP;
ROUTINE sql_error
BEGIN
 EXEC SQL WHENEVER SQLERROR CONTINUE;
 EXEC SQL ROLLBACK WORK RELEASE; 
 display 'Processing error';
 exit program with an error;
END sql_error;

Recall from Meeting Program Requirements that you can use the VARCHAR pseudotype to declare variable-length character strings. If the VARCHAR is an input host variable, you must tell Oracle what length to expect. So, set the length field to the actual length of the value stored in the string field.

If the VARCHAR is an output host variable, Oracle automatically sets the length field. However, to use a VARCHAR output host variable in your PL/SQL block, you must initialize the length field before entering the block. So, set the length field to the declared (maximum) length of the VARCHAR, as shown in the following example:

EXEC SQL BEGIN DECLARE SECTION;
 emp_number INTEGER;
 emp_name VARCHAR(10);
 salary REAL;
 ...
EXEC SQL END DECLARE SECTION;
...
set emp_name.len = 10; -- initialize length field
EXEC SQL EXECUTE
 BEGIN
 SELECT ename, sal INTO :emp_name, :salary
 FROM emp
 WHERE empno = :emp_number;
 ...
 END;
END-EXEC;

When entering a block, if an indicator variable has a value of -1, PL/SQL automatically assigns a null to the host variable. When exiting the block, if a host variable is null, PL/SQL automatically assigns a value of -1 to the indicator variable. In the next example, if handsel had a value of -1 before the PL/SQL block was entered, the salary_missing exception is raised. An exception is a named error condition.

EXEC SQL EXECUTE
 BEGIN
 IF :salary:ind_sal IS NULL THEN
 RAISE salary_missing;
 END IF;
 ...
 END;
END-EXEC;

PL/SQL does not raise an exception when a truncated string value is assigned to a host variable. However, if you use an indicator variable, PL/SQL sets it to the original length of the string. In the following example, the host program will be able to tell, by checking the value of ind_name, if a truncated value was assigned to emp_name:

EXEC SQL EXECUTE
 DECLARE
 ...
 new_name CHAR(10);
 BEGIN
 ...
 :emp_name:ind_name := new_name;
 ...
 END;
END-EXEC;

PL/SQL lets you access remote databases through database links. Typically, database links are established by your DBA and stored in the Oracle data dictionary. A database link tells Oracle where the remote database is located, the path to it, and what Oracle username and password to use. In the following example, you use the database link dallas to call the raise_salary procedure:

EXEC SQL EXECUTE
 BEGIN
 raise_salary@dallas(:emp_id, :increase);
 END;
END-EXEC;

You can create synonyms to provide location transparency for remote subprograms, as the following example shows:

CREATE PUBLIC SYNONYM raise_salary FOR raise_salary@dallas;

In Meeting Program Requirements, you learned how to embed OCI calls in your host program. After calling the library routine SQLLDA to set up the LDA, you can use the OCI call ODESSP to get useful information about a stored subprogram. When you call ODESSP, you must pass it a valid LDA and the name of the subprogram. For packaged subprograms, you must also pass the name of the package. ODESSP returns information about each subprogram parameter such as its datatype, size, position, and so on.

You can also use the procedure describe_procedure in package DBMS_DESCRIBE, which is supplied with Oracle.

In dynamic SQL Method 4, a host array cannot be bound to a PL/SQL procedure with a parameter of type TABLE.