PL/SQL - 嵌套循环

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  简述

  PL/SQL 允许在另一个循环中使用一个循环。以下部分显示了一些示例来说明这个概念。

  PL/SQL 中嵌套基本 LOOP 语句的语法如下：

  
  LOOP 
     Sequence of statements1 
     LOOP 
        Sequence of statements2 
     END LOOP; 
  END LOOP;
  

  PL/SQL 中嵌套 FOR LOOP 语句的语法如下：

  
  FOR counter1 IN initial_value1 .. final_value1 LOOP 
     sequence_of_statements1 
     FOR counter2 IN initial_value2 .. final_value2 LOOP 
        sequence_of_statements2 
     END LOOP; 
  END LOOP;
  

  Pascal 中嵌套的 WHILE LOOP 语句的语法如下：

  
  WHILE condition1 LOOP 
     sequence_of_statements1 
     WHILE condition2 LOOP 
        sequence_of_statements2 
     END LOOP; 
  END LOOP; 
  

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  例子

  以下程序使用嵌套的基本循环来查找从 2 到 100 的素数 -

  
  DECLARE 
     i number(3); 
     j number(3); 
  BEGIN 
     i := 2; 
     LOOP 
        j:= 2; 
        LOOP 
           exit WHEN ((mod(i, j) = 0) or (j = i)); 
           j := j +1; 
        END LOOP; 
     IF (j = i ) THEN 
        dbms_output.put_line(i || ' is prime'); 
     END IF; 
     i := i + 1; 
     exit WHEN i = 50; 
     END LOOP; 
  END; 
  /
  

  在 SQL 提示符下执行上述代码时，会产生以下结果 -

  
  2 is prime 
  3 is prime 
  5 is prime 
  7 is prime 
  11 is prime 
  13 is prime 
  17 is prime 
  19 is prime 
  23 is prime 
  29 is prime 
  31 is prime 
  37 is prime 
  41 is prime 
  43 is prime 
  47 is prime  
  PL/SQL procedure successfully completed.