PL/SQL基本程序结构和语句

1.条件结构

CASE

语法

CASE selector
WHEN 'value1' THEN S1;
WHEN 'value2' THEN S2;
WHEN 'value3' THEN S3;
...
ELSE Sn;  -- default case
END CASE;

实例

DECLARE
grade char(1) := 'A';
BEGIN
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('Well done');
when 'D' then dbms_output.put_line('You passed');
when 'F' then dbms_output.put_line('Better try again');
else dbms_output.put_line('No such grade');
END CASE;
END;

IF

语法

IF THEN 结构

IF condition THEN
S;
END IF;

IF THEN ELSIF ELSE 语法

IF(boolean_expression 1)THEN
S1; -- Executes when the boolean expression 1 is true
ELSIF( boolean_expression 2) THEN
S2;  -- Executes when the boolean expression 2 is true
ELSIF( boolean_expression 3) THEN
S3; -- Executes when the boolean expression 3 is true
ELSE
S4; -- executes when the none of the above condition is true
END IF;

IF THEN ELSE 语法

IF condition THEN
S1;
ELSE
S2;
END IF;

实例

IF THEN 结构实例

DECLARE
a number(2) := 10;
BEGIN
a:= 10;
-- check the boolean condition using if statement
IF( a < 20 ) THEN
-- if condition is true then print the following
dbms_output.put_line('a is less than 20 ' );
END IF;
dbms_output.put_line('value of a is : ' || a);
END;
/

IF THEN ELSIF ELSE 结构实例

DECLARE
a number(3) := 100;
BEGIN
-- check the boolean condition using if statement
IF( a < 20 ) THEN
-- if condition is true then print the following
dbms_output.put_line('a is less than 20 ' );
ELSE
dbms_output.put_line('a is not less than 20 ' );
END IF;
dbms_output.put_line('value of a is : ' || a);
END;
/

IF THEN ELSE 结构实例

DECLARE
a number(3) := 100;
BEGIN
-- check the boolean condition using if statement
IF( a < 20 ) THEN
-- if condition is true then print the following
dbms_output.put_line('a is less than 20 ' );
ELSE
dbms_output.put_line('a is not less than 20 ' );
END IF;
dbms_output.put_line('value of a is : ' || a);
END;
/

2.循环结构

LOOP

语法

LOOP
Sequence of statements;
END LOOP;

实例

DECLARE
x number := 10;
BEGIN
LOOP
dbms_output.put_line(x);
x := x + 10;
IF x > 50 THEN
exit;
END IF;
END LOOP;
-- after exit, control resumes here
dbms_output.put_line('After Exit x is: ' || x);
END;
/

FOR

语法

FOR counter IN initial_value .. final_value LOOP
sequence_of_statements;
END LOOP;

实例

DECLARE
a number(2);
BEGIN
FOR a in 10 .. 20 LOOP
dbms_output.put_line('value of a: ' || a);
END LOOP;
END;
/

3) WHILE

语法

WHILE condition LOOP
sequence_of_statements
END LOOP;

实例

DECLARE
a number(2) := 10;
BEGIN
WHILE a < 20 LOOP
dbms_output.put_line('value of a: ' || a);
a := a + 1;
END LOOP;
END;
/

3.顺序结构

GOTO

在PL/SQL编程语言的GOTO语句提供无条件跳转到在同一个子程序的GOTO标签的语句。

注意：GOTO语句是不建议使用在任何编程语言，因为它使得程序难以跟踪控制流程，使程序难以理解，难以修改。如果使用GOTO的任何程序可以改写，就尽量不要使用GOTO语句。

语法

GOTO label;
..
..
<< label >>
statement;

实例

DECLARE
a number(2) := 10;
BEGIN
<<loopstart>>
-- while loop execution
WHILE a < 20 LOOP
dbms_output.put_line ('value of a: ' || a);
a := a + 1;
IF a = 15 THEN
a := a + 1;
GOTO loopstart;
END IF;
END LOOP;
END;
/

NULL

NULL只是把控制权传递给后面的语句。

NULL语句的用处：

n 为GOTO语句提供一个目标

n 通过使条件语句的含义和行为更加清晰来提高可读性

n 创建占位符和子程序桩

n 表明清楚这种可能性，但不需要进行处理。