Oracle® Database PL/SQL User's Guide and Reference 10g Release 2 (10.2) Part Number B14261-01 |
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This chapter shows you how to structure the flow of control through a PL/SQL program. PL/SQL provides conditional tests, loops, and branches that let you produce well-structured programs.
This chapter contains these topics:
Procedural computer programs use the basic control structures shown in Figure 4-1.
The selection structure tests a condition, then executes one sequence of statements instead of another, depending on whether the condition is true or false. A condition is any variable or expression that returns a BOOLEAN
value (TRUE
or FALSE
). The iteration structure executes a sequence of statements repeatedly as long as a condition holds true. The sequence structure simply executes a sequence of statements in the order in which they occur.
The IF
statement executes a sequence of statements depending on the value of a condition. There are three forms of IF
statements: IF-THEN
, IF-THEN-ELSE
, and IF-THEN-ELSIF
. For a description of the syntax of the IF
statement, see "IF Statement".
The CASE
statement is a compact way to evaluate a single condition and choose between many alternative actions. It makes sense to use CASE
when there are three or more alternatives to choose from. For a description of the syntax of the CASE
statement, see "CASE Statement".
The simplest form of IF
statement associates a condition with a sequence of statements enclosed by the keywords THEN
and END
IF
(not ENDIF
) as illustrated in Example 4-1.
The sequence of statements is executed only if the condition is TRUE
. If the condition is FALSE
or NULL
, the IF
statement does nothing. In either case, control passes to the next statement.
Example 4-1 Using a Simple IF-THEN Statement
DECLARE sales NUMBER(8,2) := 10100; quota NUMBER(8,2) := 10000; bonus NUMBER(6,2); emp_id NUMBER(6) := 120; BEGIN IF sales > (quota + 200) THEN bonus := (sales - quota)/4; UPDATE employees SET salary = salary + bonus WHERE employee_id = emp_id; END IF; END; /The second form of IF
statement adds the keyword ELSE
followed by an alternative sequence of statements, as shown in Example 4-2.
The statements in the ELSE
clause are executed only if the condition is FALSE
or NULL
. The IF-THEN-ELSE
statement ensures that one or the other sequence of statements is executed. In the Example 4-2, the first UPDATE
statement is executed when the condition is TRUE
, and the second UPDATE
statement is executed when the condition is FALSE
or NULL
.
Example 4-2 Using a Simple IF-THEN-ELSE Statement
DECLARE sales NUMBER(8,2) := 12100; quota NUMBER(8,2) := 10000; bonus NUMBER(6,2); emp_id NUMBER(6) := 120; BEGIN IF sales > (quota + 200) THEN bonus := (sales - quota)/4; ELSE bonus := 50; END IF; UPDATE employees SET salary = salary + bonus WHERE employee_id = emp_id; END; /
IF
statements can be nested as shown in Example 4-3.
Example 4-3 Nested IF Statements
DECLARE sales NUMBER(8,2) := 12100; quota NUMBER(8,2) := 10000; bonus NUMBER(6,2); emp_id NUMBER(6) := 120; BEGIN IF sales > (quota + 200) THEN bonus := (sales - quota)/4; ELSE IF sales > quota THEN bonus := 50; ELSE bonus := 0; END IF; END IF; UPDATE employees SET salary = salary + bonus WHERE employee_id = emp_id; END; /
Sometimes you want to choose between several alternatives. You can use the keyword ELSIF
(not ELSEIF
or ELSE IF
) to introduce additional conditions, as shown in Example 4-4.
If the first condition is FALSE
or NULL
, the ELSIF
clause tests another condition. An IF
statement can have any number of ELSIF
clauses; the final ELSE
clause is optional. Conditions are evaluated one by one from top to bottom. If any condition is TRUE
, its associated sequence of statements is executed and control passes to the next statement. If all conditions are false or NULL
, the sequence in the ELSE
clause is executed, as shown in Example 4-4.
Example 4-4 Using the IF-THEN-ELSEIF Statement
DECLARE sales NUMBER(8,2) := 20000; bonus NUMBER(6,2); emp_id NUMBER(6) := 120; BEGIN IF sales > 50000 THEN bonus := 1500; ELSIF sales > 35000 THEN bonus := 500; ELSE bonus := 100; END IF; UPDATE employees SET salary = salary + bonus WHERE employee_id = emp_id; END; /
If the value of sales
is larger than 50000, the first and second conditions are TRUE
. Nevertheless, bonus
is assigned the proper value of 1500 because the second condition is never tested. When the first condition is TRUE
, its associated statement is executed and control passes to the INSERT
statement.
Another example of an IF
-THEN
-ELSE
statement is Example 4-5.
Example 4-5 Extended IF-THEN Statement
DECLARE grade CHAR(1); BEGIN grade := 'B'; IF grade = 'A' THEN DBMS_OUTPUT.PUT_LINE('Excellent'); ELSIF grade = 'B' THEN DBMS_OUTPUT.PUT_LINE('Very Good'); ELSIF grade = 'C' THEN DBMS_OUTPUT.PUT_LINE('Good'); ELSIF grade = 'D' THEN DBMS_OUTPUT. PUT_LINE('Fair'); ELSIF grade = 'F' THEN DBMS_OUTPUT.PUT_LINE('Poor'); ELSE DBMS_OUTPUT.PUT_LINE('No such grade'); END IF; ENd; /
Like the IF
statement, the CASE
statement selects one sequence of statements to execute. However, to select the sequence, the CASE
statement uses a selector rather than multiple Boolean expressions. A selector is an expression whose value is used to select one of several alternatives.
To compare the IF
and CASE
statements, consider the code in Example 4-5 that outputs descriptions of school grades. Note the five Boolean expressions. In each instance, we test whether the same variable, grade
, is equal to one of five values: 'A'
, 'B'
, 'C'
, 'D'
, or 'F'
. You can rewrite the code inExample 4-5 using the CASE
statement, as shown in Example 4-6.
Example 4-6 Using the CASE-WHEN Statement
DECLARE grade CHAR(1); BEGIN grade := 'B'; CASE grade WHEN 'A' THEN DBMS_OUTPUT.PUT_LINE('Excellent'); WHEN 'B' THEN DBMS_OUTPUT.PUT_LINE('Very Good'); WHEN 'C' THEN DBMS_OUTPUT.PUT_LINE('Good'); WHEN 'D' THEN DBMS_OUTPUT.PUT_LINE('Fair'); WHEN 'F' THEN DBMS_OUTPUT.PUT_LINE('Poor'); ELSE DBMS_OUTPUT.PUT_LINE('No such grade'); END CASE; END; /
The CASE
statement is more readable and more efficient. When possible, rewrite lengthy IF-THEN-ELSIF
statements as CASE
statements.
The CASE
statement begins with the keyword CASE
. The keyword is followed by a selector, which is the variable grade
in the last example. The selector expression can be arbitrarily complex. For example, it can contain function calls. Usually, however, it consists of a single variable. The selector expression is evaluated only once. The value it yields can have any PL/SQL datatype other than BLOB
, BFILE
, an object type, a PL/SQL record, an index-by-table, a varray, or a nested table.
The selector is followed by one or more WHEN
clauses, which are checked sequentially. The value of the selector determines which clause is executed. If the value of the selector equals the value of a WHEN
-clause expression, that WHEN
clause is executed. For instance, in the last example, if grade
equals 'C'
, the program outputs 'Good'
. Execution never falls through; if any WHEN
clause is executed, control passes to the next statement.
The ELSE
clause works similarly to the ELSE
clause in an IF
statement. In the last example, if the grade is not one of the choices covered by a WHEN
clause, the ELSE
clause is selected, and the phrase 'No such grade'
is output. The ELSE
clause is optional. However, if you omit the ELSE
clause, PL/SQL adds the following implicit ELSE
clause:
ELSE RAISE CASE_NOT_FOUND;
There is always a default action, even when you omit the ELSE
clause. If the CASE
statement does not match any of the WHEN
clauses and you omit the ELSE
clause, PL/SQL raises the predefined exception CASE_NOT_FOUND
.
The keywords END
CASE
terminate the CASE
statement. These two keywords must be separated by a space. The CASE
statement has the following form:
Like PL/SQL blocks, CASE
statements can be labeled. The label, an undeclared identifier enclosed by double angle brackets, must appear at the beginning of the CASE
statement. Optionally, the label name can also appear at the end of the CASE
statement.
Exceptions raised during the execution of a CASE
statement are handled in the usual way. That is, normal execution stops and control transfers to the exception-handling part of your PL/SQL block or subprogram.
An alternative to the CASE
statement is the CASE
expression, where each WHEN
clause is an expression. For details, see "CASE Expressions".
PL/SQL also provides a searched CASE
statement, similar to the simple CASE
statement, which has the form shown in Example 4-7.
The searched CASE
statement has no selector. Also, its WHEN
clauses contain search conditions that yield a Boolean value, not expressions that can yield a value of any type. as shown in Example 4-7.
Example 4-7 Using the Searched CASE Statement
DECLARE grade CHAR(1); BEGIN grade := 'B'; CASE WHEN grade = 'A' THEN DBMS_OUTPUT.PUT_LINE('Excellent'); WHEN grade = 'B' THEN DBMS_OUTPUT.PUT_LINE('Very Good'); WHEN grade = 'C' THEN DBMS_OUTPUT.PUT_LINE('Good'); WHEN grade = 'D' THEN DBMS_OUTPUT.PUT_LINE('Fair'); WHEN grade = 'F' THEN DBMS_OUTPUT.PUT_LINE('Poor'); ELSE DBMS_OUTPUT.PUT_LINE('No such grade'); END CASE; END; -- rather than using the ELSE in the CASE, could use the following -- EXCEPTION -- WHEN CASE_NOT_FOUND THEN -- DBMS_OUTPUT.PUT_LINE('No such grade'); /
The search conditions are evaluated sequentially. The Boolean value of each search condition determines which WHEN
clause is executed. If a search condition yields TRUE
, its WHEN
clause is executed. If any WHEN
clause is executed, control passes to the next statement, so subsequent search conditions are not evaluated.
If none of the search conditions yields TRUE
, the ELSE
clause is executed. The ELSE
clause is optional. However, if you omit the ELSE
clause, PL/SQL adds the following implicit ELSE
clause:
ELSE RAISE CASE_NOT_FOUND;
Exceptions raised during the execution of a searched CASE
statement are handled in the usual way. That is, normal execution stops and control transfers to the exception-handling part of your PL/SQL block or subprogram.
Avoid clumsy IF
statements like those in the following example:
IF new_balance < minimum_balance THEN
overdrawn := TRUE;
ELSE
overdrawn := FALSE;
END IF;
...
IF overdrawn = TRUE THEN
RAISE insufficient_funds;
END IF;
The value of a Boolean expression can be assigned directly to a Boolean variable. You can replace the first IF
statement with a simple assignment:
overdrawn := new_balance < minimum_balance;
A Boolean variable is itself either true or false. You can simplify the condition in the second IF
statement:
IF overdrawn THEN ...
When possible, use the ELSIF
clause instead of nested IF
statements. Your code will be easier to read and understand. Compare the following IF
statements:
IF condition1 THEN statement1;
ELSE IF condition2 THEN statement2;
ELSE IF condition3 THEN statement3; END IF;
END IF;
END IF;
IF condition1 THEN statement1;
ELSEIF condition2 THEN statement2;
ELSEIF condition3 THEN statement3;
END IF;
These statements are logically equivalent, but the second statement makes the logic clearer.
To compare a single expression to multiple values, you can simplify the logic by using a single CASE
statement instead of an IF
with several ELSIF
clauses.
LOOP
statements execute a sequence of statements multiple times. There are three forms of LOOP
statements: LOOP
, WHILE-LOOP
, and FOR-LOOP
. For a description of the syntax of the LOOP
statement, see "LOOP Statements".
The simplest form of LOOP
statement is the basic loop, which encloses a sequence of statements between the keywords LOOP
and END
LOOP
, as follows:
LOOP
END LOOP;
With each iteration of the loop, the sequence of statements is executed, then control resumes at the top of the loop. You use an EXIT
statement to stop looping and prevent an infinite loop. You can place one or more EXIT
statements anywhere inside a loop, but not outside a loop. There are two forms of EXIT
statements: EXIT
and EXIT-WHEN
.
The EXIT
statement forces a loop to complete unconditionally. When an EXIT
statement is encountered, the loop completes immediately and control passes to the next statement as shown in Example 4-8.
Example 4-8 Using an EXIT Statement
DECLARE credit_rating NUMBER := 0; BEGIN LOOP credit_rating := credit_rating + 1; IF credit_rating > 3 THEN EXIT; -- exit loop immediately END IF; END LOOP; -- control resumes here DBMS_OUTPUT.PUT_LINE ('Credit rating: ' || TO_CHAR(credit_rating)); IF credit_rating > 3 THEN RETURN; -- use RETURN not EXIT when outside a LOOP END IF; DBMS_OUTPUT.PUT_LINE ('Credit rating: ' || TO_CHAR(credit_rating)); END; /
Remember, the EXIT
statement must be placed inside a loop. To complete a PL/SQL block before its normal end is reached, you can use the RETURN
statement. For more information, see "Using the RETURN Statement".
The EXIT-WHEN
statement lets a loop complete conditionally. When the EXIT
statement is encountered, the condition in the WHEN
clause is evaluated. If the condition is true, the loop completes and control passes to the next statement after the loop. See Example 1-10 for an example that uses the EXIT-WHEN
statement.
Until the condition is true, the loop cannot complete. A statement inside the loop must change the value of the condition. In the previous example, if the FETCH
statement returns a row, the condition is false. When the FETCH
statement fails to return a row, the condition is true, the loop completes, and control passes to the CLOSE
statement.
The EXIT-WHEN
statement replaces a simple IF
statement. For example, compare the following statements:
IF count > 100 THEN EXIT; ENDIF;
EXIT WHEN count > 100;
These statements are logically equivalent, but the EXIT-WHEN
statement is easier to read and understand.
Like PL/SQL blocks, loops can be labeled. The optional label, an undeclared identifier enclosed by double angle brackets, must appear at the beginning of the LOOP
statement. The label name can also appear at the end of the LOOP
statement. When you nest labeled loops, use ending label names to improve readability.
With either form of EXIT
statement, you can complete not only the current loop, but any enclosing loop. Simply label the enclosing loop that you want to complete. Then, use the label in an EXIT
statement, as shown in Example 4-9. Every enclosing loop up to and including the labeled loop is exited.
Example 4-9 Using EXIT With Labeled Loops
DECLARE s PLS_INTEGER := 0; i PLS_INTEGER := 0; j PLS_INTEGER; BEGIN <<outer_loop>> LOOP i := i + 1; j := 0; <<inner_loop>> LOOP j := j + 1; s := s + i * j; -- sum a bunch of products EXIT inner_loop WHEN (j > 5); EXIT outer_loop WHEN ((i * j) > 15); END LOOP inner_loop; END LOOP outer_loop; DBMS_OUTPUT.PUT_LINE('The sum of products equals: ' || TO_CHAR(s)); END; /
The WHILE-LOOP
statement executes the statements in the loop body as long as a condition is true:
WHILE
condition LOOP
sequence_of_statementsEND
LOOP;
Before each iteration of the loop, the condition is evaluated. If it is TRUE
, the sequence of statements is executed, then control resumes at the top of the loop. If it is FALSE
or NULL
, the loop is skipped and control passes to the next statement. See Example 1-9 for an example using the WHILE-LOOP
statement.
The number of iterations depends on the condition and is unknown until the loop completes. The condition is tested at the top of the loop, so the sequence might execute zero times.
Some languages have a LOOP
UNTIL
or REPEAT
UNTIL
structure, which tests the condition at the bottom of the loop instead of at the top, so that the sequence of statements is executed at least once. The equivalent in PL/SQL would be:
LOOP
sequence_of_statements EXIT
WHEN
boolean_expression;END
LOOP;
To ensure that a WHILE
loop executes at least once, use an initialized Boolean variable in the condition, as follows:
done := FALSE;
WHILE NOT done LOOP
sequence_of_statements done :=
boolean_expression;END LOOP;
A statement inside the loop must assign a new value to the Boolean variable to avoid an infinite loop.
Simple FOR
loops iterate over a specified range of integers. The number of iterations is known before the loop is entered. A double dot (..
) serves as the range operator. The range is evaluated when the FOR
loop is first entered and is never re-evaluated. If the lower bound equals the higher bound, the loop body is executed once.
As Example 4-10 shows, the sequence of statements is executed once for each integer in the range 1
to 500
. After each iteration, the loop counter is incremented.
Example 4-10 Using a Simple FOR..LOOP Statement
DECLARE p NUMBER := 0; BEGIN FOR k IN 1..500 LOOP -- calculate pi with 500 terms p := p + ( ( (-1) ** (k + 1) ) / ((2 * k) - 1) ); END LOOP; p := 4 * p; DBMS_OUTPUT.PUT_LINE( 'pi is approximately : ' || p ); -- print result END; /
By default, iteration proceeds upward from the lower bound to the higher bound. If you use the keyword REVERSE
, iteration proceeds downward from the higher bound to the lower bound. After each iteration, the loop counter is decremented. You still write the range bounds in ascending (not descending) order.
Example 4-11 Using a Reverse FOR..LOOP Statement
BEGIN FOR i IN REVERSE 1..3 LOOP -- assign the values 1,2,3 to i DBMS_OUTPUT.PUT_LINE (TO_CHAR(i)); END LOOP; END; /
Inside a FOR
loop, the counter can be read but cannot be changed.
BEGIN FOR i IN 1..3 LOOP -- assign the values 1,2,3 to i IF i < 3 THEN DBMS_OUTPUT.PUT_LINE (TO_CHAR(i)); ELSE i := 2; -- not allowed, raises an error END IF; END LOOP; END; /
A useful variation of the FOR
loop uses a SQL query instead of a range of integers. This technique lets you run a query and process all the rows of the result set with straightforward syntax. For details, see "Querying Data with PL/SQL: Implicit Cursor FOR Loop".
The bounds of a loop range can be literals, variables, or expressions but must evaluate to numbers. Otherwise, PL/SQL raises the predefined exception VALUE_ERROR
. The lower bound need not be 1, but the loop counter increment or decrement must be 1.
j IN -5..5
k IN REVERSE first..last
step IN 0..TRUNC(high/low) * 2
Internally, PL/SQL assigns the values of the bounds to temporary PLS_INTEGER
variables, and, if necessary, rounds the values to the nearest integer. The magnitude range of a PLS_INTEGER
is -2147483648 to 2147483647, represented in 32 bits. If a bound evaluates to a number outside that range, you get a numeric overflow error when PL/SQL attempts the assignment. See "PLS_INTEGER Datatype".
Some languages provide a STEP
clause, which lets you specify a different increment (5 instead of 1 for example). PL/SQL has no such structure, but you can easily build one. Inside the FOR
loop, simply multiply each reference to the loop counter by the new increment. In Example 4-12, you assign today's date to elements 5, 10, and 15 of an index-by table:
Example 4-12 Changing the Increment of the Counter in a FOR..LOOP Statement
DECLARE TYPE DateList IS TABLE OF DATE INDEX BY PLS_INTEGER; dates DateList; k CONSTANT INTEGER := 5; -- set new increment BEGIN FOR j IN 1..3 LOOP dates(j*k) := SYSDATE; -- multiply loop counter by increment END LOOP; END; /
PL/SQL lets you specify the loop range at run time by using variables for bounds as shown in Example 4-13.
Example 4-13 Specifying a LOOP Range at Run Time
CREATE TABLE temp (emp_no NUMBER, email_addr VARCHAR2(50)); DECLARE emp_count NUMBER; BEGIN SELECT COUNT(employee_id) INTO emp_count FROM employees; FOR i IN 1..emp_count LOOP INSERT INTO temp VALUES(i, 'to be added later'); END LOOP; COMMIT; END; /
If the lower bound of a loop range evaluates to a larger integer than the upper bound, the loop body is not executed and control passes to the next statement:
FOR i IN 2..limit LOOP
sequence_of_statements -- executes zero timesEND LOOP;
The loop counter is defined only within the loop. You cannot reference that variable name outside the loop. After the loop exits, the loop counter is undefined:
Example 4-14 Scope of the LOOP Counter Variable
BEGIN FOR i IN 1..3 LOOP -- assign the values 1,2,3 to i DBMS_OUTPUT.PUT_LINE (TO_CHAR(i)); END LOOP; DBMS_OUTPUT.PUT_LINE (TO_CHAR(i)); -- raises an error END; /
You do not need to declare the loop counter because it is implicitly declared as a local variable of type INTEGER
. It is safest not to use the name of an existing variable, because the local declaration hides any global declaration.
DECLARE i NUMBER := 5; BEGIN FOR i IN 1..3 LOOP -- assign the values 1,2,3 to i DBMS_OUTPUT.PUT_LINE (TO_CHAR(i)); END LOOP; DBMS_OUTPUT.PUT_LINE (TO_CHAR(i)); -- refers to original variable value (5) END; /
To reference the global variable in this example, you must use a label and dot notation, as shown in Example 4-15.
Example 4-15 Using a Label for Referencing Variables Outside a Loop
<<main>> DECLARE i NUMBER := 5; BEGIN FOR i IN 1..3 LOOP -- assign the values 1,2,3 to i DBMS_OUTPUT.PUT_LINE( 'local: ' || TO_CHAR(i) || ' global: ' || TO_CHAR(main.i)); END LOOP; END main; /
The same scope rules apply to nested FOR
loops. In Example 4-16 both loop counters have the same name. To reference the outer loop counter from the inner loop, you use a label and dot notation.
Example 4-16 Using Labels on Loops for Referencing
BEGIN <<outer_loop>> FOR i IN 1..3 LOOP -- assign the values 1,2,3 to i <<inner_loop>> FOR i IN 1..3 LOOP IF outer_loop.i = 2 THEN DBMS_OUTPUT.PUT_LINE( 'outer: ' || TO_CHAR(outer_loop.i) || ' inner: ' || TO_CHAR(inner_loop.i)); END IF; END LOOP inner_loop; END LOOP outer_loop; END; /
The EXIT
statement lets a FOR
loop complete early. In Example 4-17, the loop normally executes ten times, but as soon as the FETCH
statement fails to return a row, the loop completes no matter how many times it has executed.
Example 4-17 Using EXIT in a LOOP
DECLARE v_employees employees%ROWTYPE; -- declare record variable CURSOR c1 is SELECT * FROM employees; BEGIN OPEN c1; -- open the cursor before fetching -- An entire row is fetched into the v_employees record FOR i IN 1..10 LOOP FETCH c1 INTO v_employees; EXIT WHEN c1%NOTFOUND; -- process data here END LOOP; CLOSE c1; END; /
Suppose you must exit early from a nested FOR
loop. To complete not only the current loop, but also any enclosing loop, label the enclosing loop and use the label in an EXIT
statement as shown in Example 4-18.
Example 4-18 Using EXIT With a Label in a LOOP
DECLARE v_employees employees%ROWTYPE; -- declare record variable CURSOR c1 is SELECT * FROM employees; BEGIN OPEN c1; -- open the cursor before fetching -- An entire row is fetched into the v_employees record <<outer_loop>> FOR i IN 1..10 LOOP -- process data here FOR j IN 1..10 LOOP FETCH c1 INTO v_employees; EXIT WHEN c1%NOTFOUND; -- process data here END LOOP; END LOOP outer_loop; CLOSE c1; END; /
See also Example 6-10.
Unlike the IF
and LOOP
statements, the GOTO
and NULL
statements are not crucial to PL/SQL programming. The GOTO
statement is seldom needed. Occasionally, it can simplify logic enough to warrant its use. The NULL
statement can improve readability by making the meaning and action of conditional statements clear.
Overuse of GOTO
statements can result in code that is hard to understand and maintain. Use GOTO
statements sparingly. For example, to branch from a deeply nested structure to an error-handling routine, raise an exception rather than use a GOTO
statement. PL/SQL's exception-handling mechanism is discussed in Chapter 10, "Handling PL/SQL Errors".
The GOTO
statement branches to a label unconditionally. The label must be unique within its scope and must precede an executable statement or a PL/SQL block. When executed, the GOTO
statement transfers control to the labeled statement or block. The labeled statement or block can be down or up in the sequence of statements. In Example 4-19 you go to a PL/SQL block up in the sequence of statements.
Example 4-19 Using a Simple GOTO Statement
DECLARE p VARCHAR2(30); n PLS_INTEGER := 37; -- test any integer > 2 for prime BEGIN FOR j in 2..ROUND(SQRT(n)) LOOP IF n MOD j = 0 THEN -- test for prime p := ' is not a prime number'; -- 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; /
The label end_loop
in the Example 4-20 is not allowed unless it is preceded by an executable statement. To make the label legal, a NULL
statement is added.
Example 4-20 Using a NULL Statement to Allow a GOTO to a Label
DECLARE done BOOLEAN; BEGIN FOR i IN 1..50 LOOP IF done THEN GOTO end_loop; END IF; <<end_loop>> -- not allowed unless an executable statement follows NULL; -- add NULL statement to avoid error END LOOP; -- raises an error without the previous NULL END; /
Example 4-21 shows a GOTO
statement can branch to an enclosing block from the current block.
Example 4-21 Using a GOTO Statement to Branch an Enclosing Block
-- example with GOTO statement DECLARE v_last_name VARCHAR2(25); v_emp_id NUMBER(6) := 120; BEGIN <<get_name>> SELECT last_name INTO v_last_name FROM employees WHERE employee_id = v_emp_id; BEGIN DBMS_OUTPUT.PUT_LINE (v_last_name); v_emp_id := v_emp_id + 5; IF v_emp_id < 120 THEN GOTO get_name; -- branch to enclosing block END IF; END; END; /
The GOTO
statement branches to the first enclosing block in which the referenced label appears.
Some possible destinations of a GOTO
statement are not allowed. Specifically, a GOTO
statement cannot branch into an IF
statement, CASE
statement, LOOP
statement, or sub-block. For example, the following GOTO
statement is not allowed:
BEGIN
GOTO update_row;
-- cannot branch into IF statement IF valid THEN
<<update_row>>
UPDATE emp SET
... END IF;
END;
A GOTO
statement cannot branch from one IF
statement clause to another, or from one CASE
statement WHEN
clause to another.
A GOTO
statement cannot branch from an outer block into a sub-block (that is, an inner BEGIN-END
block).
A GOTO
statement cannot branch out of a subprogram. To end a subprogram early, you can use the RETURN
statement or use GOTO
to branch to a place right before the end of the subprogram.
A GOTO
statement cannot branch from an exception handler back into the current BEGIN-END
block. However, a GOTO
statement can branch from an exception handler into an enclosing block.
The NULL
statement does nothing, and passes control to the next statement. Some languages refer to such an instruction as a no-op (no operation). See "NULL Statement".
In Example 4-22, the NULL
statement emphasizes that only salespeople receive commissions.
Example 4-22 Using the NULL Statement to Show No Action
DECLARE v_job_id VARCHAR2(10); v_emp_id NUMBER(6) := 110; BEGIN SELECT job_id INTO v_job_id FROM employees WHERE employee_id = v_emp_id; IF v_job_id = 'SA_REP' THEN UPDATE employees SET commission_pct = commission_pct * 1.2; ELSE NULL; -- do nothing if not a sales representative END IF; END; /
The NULL
statement is a handy way to create placeholders and stub procedures. In Example 4-23, the NULL
statement lets you compile this procedure, then fill in the real body later. Note that the use of the NULL
statement might raise an unreachable
code
warning if warnings are enabled. See "Overview of PL/SQL Compile-Time Warnings".
Example 4-23 Using NULL as a Placeholder When Creating a Subprogram
CREATE OR REPLACE PROCEDURE award_bonus (emp_id NUMBER, bonus NUMBER) AS BEGIN -- executable part starts here NULL; -- use NULL as placeholder, raises "unreachable code" if warnings enabled END award_bonus; /
You can use the NULL
statement to indicate that you are aware of a possibility, but no action is necessary. In the following exception block, the NULL
statement shows that you have chosen not to take any action for unnamed exceptions:
EXCEPTION
WHEN ZERO_DIVIDE THEN
ROLLBACK;
WHEN OTHERS THEN
NULL;
END;