Control Structures
The Control structures are the most important PL/SQL extension to the SQL. Not only does PL/SQL let you manipulate Oracle data, it lets you process the data using iterative, conditional, and sequential flow-of-control statements like IF-THEN-ELSE, WHILE-LOOP, FOR-LOOP, EXIT-WHEN, and GOTO. Together, these statements can handle any situation.
Conditional Control
Frequently, it is necessary to take alternative actions depending on the circumstances. The IF THEN-ELSE statement executes a sequence of statements conditionally. The IF clause checks the condition; the THEN clause defines what to do if the condition is true; the ELSE clause defines what to do when the condition is false or null.
Consider the program below, that process a bank transaction. Before permitting you to withdraw $500 from account 3, it makes sure that the account has sufficient funds to cover the withdrawal. If the fund is available, the program debit the account. If not, the program inserts a record into an audit table.
-- available online in file 'examp2'
DECLARE
acct_balance NUMBER(11,2);
acct CONSTANT NUMBER(4) := 3;
debit_amt CONSTANT NUMBER(5,2) := 500.00;
BEGIN
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;
A sequence of statements that uses query results to select an alternative action is common in database applications. Another common sequence inserts/deletes a row only if an associated entry is found in other table. You can pack these common sequences into a PL/SQL block using conditional logic. This can improve the performance and simplify the integrity checks built into Oracle Forms applications.
Iterative Control
The LOOP statements execute a sequence of statements multiple times. You put the keyword LOOP before the first statement in the sequence and the keywords END LOOP after the last statement in the sequence. The example below shows the simplest kind of loop, that repeats a sequence of statements repeatedly:
LOOP
-- sequence of statements
END LOOP;
The FOR-LOOP statement specifies a range of integers, after that execute a sequence of statements once for every integer in the range. For e.g., assume that you are a producer of custom-made cars and that each car has a serial number. To keep the track of which customer buys each car, you might use the FOR loop as shown:
FOR i IN 1..order_qty LOOP
UPDATE sales SET custno = customer_id
WHERE serial_num = serial_num_seq.NEXTVAL;
END LOOP;
The WHILE-LOOP statement associates a condition with a series of statements. Before every iteration of the loop, the condition is calculated. When the condition is true, the chain of statements is executed, afterward control resumes at the top of the loop. And if the condition is false or null, the loop is bypassed and control passes to the next statement.
In the example below, you find the first employee who has a salary over $4000 and is higher in the chain of the command than employee 7902:
-- available online in file 'examp3'
DECLARE
salary emp.sal%TYPE;
mgr_num emp.mgr%TYPE;
last_name emp.ename%TYPE;
starting_empno CONSTANT NUMBER(4) := 7902;
BEGIN
SELECT sal, mgr INTO salary, mgr_num FROM emp
WHERE empno = starting_empno;
WHILE salary < 4000 LOOP
SELECT sal, mgr, ename INTO salary, mgr_num, last_name
FROM emp WHERE empno = mgr_num;
END LOOP;
INSERT INTO temp VALUES (NULL, salary, last_name);
COMMIT;
END;
The EXIT-WHEN statement completes a loop if further processing is not possible or undesirable. When the EXIT statement is encountered, the condition in the WHEN clause is checked. If the condition is true, the loop completes and control passes to the next statement. In the example below, the loop completes when the value of total exceeds 25,000:
LOOP
...
total := total + salary;
EXIT WHEN total > 25000; -- exit loop if condition is true
END LOOP;
-- control resumes here
Sequential Control
The GOTO statement branch to an unconditionally label. The label, an undeclared identifier enclosed by double angle brackets, should precede an executable statement or a PL/SQL block. If executed, the GOTO statement transfers the control to the labeled statement or block, as shown:
IF rating > 90 THEN
GOTO calc_raise; -- branch to label
END IF;
...
<>
IF job_title = 'SALESMAN' THEN -- control resumes here
amount := commission * 0.25;
ELSE
amount := salary * 0.10;
END IF;