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4 PL/SQL Control Statements

2019-10-17 21:45 1166 查看

PL/SQL has three categories of control statements:

(1) Conditional selection statements, which run different statements for different data values.(IF & CASE).

(2) Loop statements,which run the same statements with a series of different data values.(the basic LOOP,FOR LOOP, and WHILE LOOP).

(3) Sequential control statements,which are not crucial to PL/SQL programming. (GOTO)

Conditional Selection Statements

The IF statement either runs or skips a sequence of one or more statements,depending on a condition. The IF statement has these forms:

IF THEN

IF THEN ELSE

IF THEN ELSIF

IF THEN Statement

The IF THEN statement has this structure:

IF condition THEN

statements

END IF;

In Example 4–1, the statements between THEN and END IF run if and only if the value of sales is greater than quota+200.

Example 4-1 IF THEN Statement

DECLARE
PROCEDURE p
(sales NUMBER,
quota NUMBER,
emp_id NUMBER
)
IS
bonus NUMBER := 0;
updated VARCHAR2(3) := 'No';
BEGIN
IF sales > (quota + 200) THEN
bonus := (sales - quota)/4;

UPDATE employees
SET salary = salary + bonus
WHERE employee_id = emp_id;

updated := 'Yes';
END IF;

DBMS_OUTPUT.PUT_LINE('Table updated? ' || updated || ', ' ||
'bonus = ' || bonus || '.');
END p;
BEGIN
p(10100, 10000, 120);
p(10500, 10000, 121);
END;
/

IF THEN ELSE Statement

The IF THEN ELSE statement has this structure:

IF condition THEN

statements

ELSE

else_statements

END IF;

In Example 4–2, the statement between THEN and ELSE runs if and only if the value of sales is greater than quota+200; otherwise, the statement between ELSE and END IF runs.

Example 4-2 IF THEN ELSE Statement

DECLARE
PROCEDURE P(SALES NUMBER, QUOTA NUMBER, EMP_ID NUMBER) IS
BONUS NUMBER := 0;
BEGIN
IF SALES > (QUOTA + 200) THEN
BONUS := (SALES - QUOTA) / 4;
ELSE
BONUS := 50;
END IF;

DBMS_OUTPUT.PUT_LINE('bonus = ' || bonus);

UPDATE employees
SET salary = salary + bonus
WHERE employee_id = emp_id;
END P;
BEGIN
P(10100, 10000, 120);
P(10500, 10000, 121);
END;
/

Example 4-3 Nested IF THEN ELSE Statements

DECLARE
PROCEDURE p (
sales NUMBER,
quota NUMBER,
emp_id NUMBER)
IS
bonus NUMBER := 0;
BEGIN
IF sales > (quota + 200) THEN
bonus := (sales - quota)/4;
ELSE
IF sales > quota THEN
bonus := 50;
ELSE
bonus := 0;
END IF;
END IF;

DBMS_OUTPUT.PUT_LINE('bonus = ' || bonus);

UPDATE employees
SET salary = salary + bonus
WHERE employee_id = emp_id;

END p;
BEGIN
P(10100,10000,120);
p(10500,10000,121);
p(9500, 10000,122);
END;
/

IF THEN ELSIF Statement

The IF THEN ELSIF statement has this structure:

IF condition_1 THEN
statements_1
ELSIF condition_2 THEN
statements_2
[ ELSIF condition_3 THEN
statements_3
]...
[ ELSE
else_statements
]
END IF;

Example 4-4 IF THEN ELSIF Statement

DECLARE
PROCEDURE P(sales NUMBER) IS
bonus NUMBER := 0;
BEGIN
IF sales > 50000 THEN
bonus := 1500;
ELSIF sales > 35000 THEN
bonus := 500;
ELSE
bonus := 100;
END IF;

DBMS_OUTPUT.put_line('Sales = ' || sales || ', bonus = ' || bonus || '.');
END P;

BEGIN
P(55000);
P(40000);
P(30000);
END;
/

Example 4–5 IF THEN ELSIF Statement Simulates Simple CASE Statement

Simple CASE Statement

Example 4–6 Simple CASE Statement

Searched CASE Statement

Example 4–7 Searched CASE Statement

Example 4–8 EXCEPTION Instead of ELSE Clause in CASE Statement

LOOP Statements

Loop statements run the same statements with a series of different values. The loop statement are:

Basic LOOP

FOR LOOP

Cursor FOR LOOP

WHILE LOOP

The statements that exit a loop are :

EXIT

EXIT WHEN

The statements that exit the current iteration of a loop are:

CONTINUE

CONTINUE WHEN

Basic LOOP Statement

The basic LOOP statement has this structure:

[ label ] LOOP

statements

END LOOP [ label ];

EXIT Statement

Example 4-9 Basic LOOP Statement with EXIT Statement

EXIT WHEN Statement

Example 4–10 Basic LOOP Statement with EXIT WHEN Statement

Example 4–11 Nested, Labeled Basic LOOP Statements with EXIT WHEN Statements

Example 4–12 Nested, Unabeled Basic LOOP Statements with EXIT WHEN Statements

CONTINUE Statement

Example 4–13 CONTINUE Statement in Basic LOOP Statement

CONTINUE WHEN Statement

Example 4–14 CONTINUE WHEN Statement in Basic LOOP Statement

FOR LOOP Statement

The FOR LOOP statement runs one or more statements while the loop index is in a specified range. The statement has this structrure:

[label] FOR index IN [ REVERSE ] lower_bound..upper_bound LOOP

statements

END LOOP [label];

Example 4-15 FOR LOOP Statements

Example 4-16 Reverse FOR LOOP Statements

Example 4 - 17 Simulating STEP Clause in FOR LOOP Statement

FOR LOOP Index

The index of a FOR LOOP statement is implicitly declared as a variable of type PLS_INTEGER that is local to the loop. The

statements in the loop can read the value of the index.but cannot change it. Statements outside the loop cannot reference the index. After the FOR LOOP statement runs, the index is undefined.(A loop index is sometimes called a loop counter.)

Example 4–18 FOR LOOP Statement Tries to Change Index Value

Example 4–19 Outside Statement References FOR LOOP Statement Index

Example 4–20 FOR LOOP Statement Index with Same Name as Variable

Example 4–22 Nested FOR LOOP Statements with Same Index Name

WHILE LOOP Statement

The WHILE LOOP statement runs one or more statements while condition is true.It has this structure:

[ label ] WHILE condition LOOP

statements

END LOOP [ label ];

SequentiaL Control Statements

Unlike the IF and LOOP statements , the sequential control statements GOTO and NULL

are not crucial to PL/SQL programming.

GOTO Statement

The GOTO statement transfers control to a label unconditionally. The label must be unqiue in its scope and must precede an executable statement or a PL/SQL block. When run,the GOTO statement transfers control to the labeled statement or block.

Example 4-29 GOTO Statement

DECLARE
p varchar2(30);
n PLS_INTEGER := 37;
BEGIN
FOR j in 2 ..ROUND(SQRT(n)) LOOP
IF n MOD j = 0 THEN
p := ' is not a prime number';
GOTO print_now;
END IF;
END LOOP;

p := ' is a prime number';

<<print_now>>
DBMS_OUTPUT.PUT_LINE(TO_CHAR(n)|| p);
END;
/