[bbk4982]第09集 Chapter 05 Writing Control Structures(01)

Logic Tables

Build a simple Boolean condition with a comparision operator.

AND	TRUE	FALSE	NULL
TRUE	TRUE	FALSE	NULL
FALSE	FALSE	FALSE	FALSE
NULL	NULL	FALSE	NULL

　　　　　　

OR	TRUE	FALSE	NULL
TRUE	TRUE	TRUE	TRUE
FALSE	TRUE	FALSE	NULL
NULL	TRUE	NULL	NULL

NOT
TRUE	FALSE
FALSE	TRUE
NULL	NULL

Boolean Expression or Logincal Expression 

What is the value of flag in each case?

flag := record_flag AND available_flag;

RECORD_FLAG	AVAILABLE_FLAG	FLAG
TRUE	TRUE	?(1)
TRUE	FALSE	?(2)
NULL	TRUE	?(3)
NULL	FALSE	?(4)

Short-Circuit Evaluation

PL/SQL uses short-circuit evaluation,which means that PL/SQL need not evaluate all of the expression in an IF statements. 

如果运算符号是AND，FALSE具有黑洞特色；

如果运算符号是OR,TRUE具有黑洞特色;

IF condition1 AND condition2
THEN
    statements;
ELSE
    statements;
END IF;

IF condition1 OR condition2
THEN
    statements;
ELSE
    statements;
END IF;

Demo01:如果condition1计算结果是FALSE,那么condition2就不用计算.

Demo02:如果condition1计算结果是TRUE,那么condition2就不用计算.

Quiz

Is the following statement short-circuit when condition1 is NULL,when condition1 is FALSE?

my_boolean := condtion1 AND condition2;

The NULL Statement

NULL除了可以作为一个表达式之外，还可以作为一个Statement.

NULL expression;

NULL Statement;

Syntax:

--NULL Statement;
NULL;

When you want PL/SQL to do absolutely nothing,you can use the NULL statements.The reasons to the use NULL statement are often the two following scenarios:

• Improving program readability.
• Using the NULL statement after a label.(oracle 语法规定，标签必须在语句之前.)

  • DECLARE
    ...
    BEGIN
        IF condition THEN GOTO lastpoint END IF;
        ...
        ...
        ...
    <<lastpoint>>
        NULL;
    END;
    /

Iterative Control:Loop Statements

• Loops repeat a statement(or a sequence of statements) multiple times.
• There are three loop types:
  • Basic loop
  • FOR loop
  • WHILE loop

Basic Loops

Syntax:

LOOP
    statement1;
    ...
    ...
    ...
    EXIT [WHEN condition];
END LOOP;

Basic Loops中如果没有EXIT ，就将会是一个死循环.

DECLARE
        v_countryid     loc.country_id%TYPE := 'CA';
        v_loc_id        loc.location_id%TYPE;
        v_counter       NUMBER(2) := 1;
        v_new_city      loc.city%TYPE := 'Montreal';
BEGIN
        SELECT MAX(location_id) INTO v_loc_id   FROM loc WHERE country_id = v_countryid;
        LOOP
                INSERT INTO loc(location_id,city,country_id)
                VALUES((v_loc_id + v_counter),v_new_city,v_countryid);

                v_counter := v_counter + 1;

                EXIT WHEN v_counter > 3;

        END LOOP;

        COMMIT;
EXCEPTION
        WHEN OTHERS THEN
                ROLLBACK;
                DBMS_OUTPUT.PUT_LINE('Error Occured;');
END;

/

SELECT location_id,city,country_id FROM loc;

WHILE LOOPS

Syntax:

WHILE condition LOOP
    statement1;
    statement2;
END LOOP;

Use the WHILE loop to repeat statements while a condition is TRUE.

DECLARE
        v_countryid     loc.country_id%TYPE := 'CA';
        v_loc_id        loc.location_id%TYPE;
        v_counter       NUMBER(2) := 1;
        v_new_city      loc.city%TYPE := 'alta';
BEGIN
        SELECT MAX(location_id) INTO v_loc_id   FROM loc WHERE country_id = v_countryid;

        /*
        LOOP
                INSERT INTO loc(location_id,city,country_id) VALUES((v_loc_id + v_counter),v_new_city,v_countryid);

                v_counter := v_counter + 1;

                EXIT WHEN v_counter > 3;

        END LOOP;
        */

        WHILE v_counter < 4
        LOOP
                INSERT INTO loc(location_id,city,country_id) VALUES((v_loc_id + v_counter),v_new_city,v_countryid);
                v_counter := v_counter + 1;
END LOOP;

        COMMIT;
EXCEPTION
        WHEN OTHERS THEN
                ROLLBACK;
                DBMS_OUTPUT.PUT_LINE('Error Occured;');
END;

/

SELECT location_id,city,country_id FROM loc;

FOR Loops(PL/SQL中默认的步长是1，不可修改)

Use a For loop to shortcut the test for the number of iterations.

Do not declare the counter;it is declared implicity.

FOR counter IN [REVERSE] lower_bound..upper_bound 
LOOP
    statement1;
    statement2;
    ...
END LOOP;

BEGIN
        FOR i IN REVERSE 1..10
        LOOP
                DBMS_OUTPUT.PUT_LINE(i);
        END LOOP;
END;

/

DECLARE
        v_countryid     loc.country_id%TYPE := 'CA';
        v_loc_id        loc.location_id%TYPE;
        v_counter       NUMBER(2) := 1;
        v_new_city      loc.city%TYPE := 'Montreal';
BEGIN
        SELECT MAX(location_id) INTO v_loc_id   FROM loc WHERE country_id = v_countryid;
        /*
        LOOP
                INSERT INTO loc(location_id,city,country_id)
                VALUES((v_loc_id + v_counter),v_new_city,v_countryid);

                v_counter := v_counter + 1;

                EXIT WHEN v_counter > 3;

        END LOOP;
        */

        FOR i IN 1..3
        LOOP
                INSERT INTO loc(location_id,city,country_id) VALUES((v_loc_id + v_counter),v_new_city,v_countryid);

                v_counter := v_counter + 1;

        END LOOP;


        COMMIT;
EXCEPTION
        WHEN OTHERS THEN
                ROLLBACK;
                DBMS_OUTPUT.PUT_LINE('Error Occured;');
END;

/

SELECT location_id,city,country_id FROM loc;