pl/sql 实例精解 05

本章主要讨论 case 语句

   1:  case [SELECTOR]

   2:    when expression 1 then statement 1;

   3:    when expression 2 then statement 2;

   4:    ...

   5:    when expression N then statement N;

   6:    else statement N+1;

   7:  end case;

注意: pl/sql 中的 case 没有 break;

   1:  declare

   2:      v_num        number := &sv_user_num;

   3:      v_num_flag    number;

   4:  begin

   5:      v_num_flage := mod(v_num, 2);

   6:      case v_num_flag

   7:          when 0 then

   8:              dbms_output.put_line(v_num || 'is even number');

   9:          else

  10:              dbms_output.put_line(v_num || 'is odd number');

  11:      end case;

  12:      dbms_output.put_line('Done');

  13:  end;

  14:  /

  15:  show errors;

注意： case 语句有两种形式,

1. 普通形式的 case 语句, 如上例 (有选择器, SELECTOR)

2. 搜索形式的 case 语句, 即 case 关键词后边没有跟表达式, 而是直接逻辑判断 when 后边的 expression , 如果为真就执行, 如下例子: (没有选择器, SELECTOR)

   1:  -- 不带选择器的(SELECTOR)CASE语句

   2:  declare

   3:      v_num        number := &sv_user_num;

   4:      v_num_flag    number;

   5:  begin

   6:      case 

   7:          when MOD(v_num, 2) = 0 then

   8:              dbms_output.put_line(v_num || 'is even number');

   9:          else

  10:              dbms_output.put_line(v_num || 'is odd number');

  11:      end case;

  12:      dbms_output.put_line('Done');

  13:  end;

  14:  /

  15:  show errors;

使用 case 表达式 (表达式一定要与变量对应)

   1:  -- case 表达式

   2:  declare

   3:      v_num        number := &sv_user_num;

   4:      v_num_flag     number;

   5:      v_result    varchar2(30);

   6:  begin

   7:      v_num_flag := mod(v_num, 2);

   8:      

   9:      v_result :=

  10:      case v_num_flag

  11:          when 0 then v_num || 'is even number'

  12:          else v_num || ' is odd number'

  13:      end;        -- 注意: 这只是 end, 而不是 end case

  14:      

  15:  end;

  16:  /

  17:  show errors;

BULLIF, COALESCE 函数

NULLIF(expression1, expression2), 如果 expression1 = expression2, 返回 null, 否则 返回 expression1. 这个函数与 NVL 函数相反

例如:

   1:  declare

   2:      v_num        number := &sv_user_num;

   3:      v_remainder    number;

   4:  begin

   5:      v_remainder := NULLIF(MOD(v_num, 2), 0);  -- 函数会返回值

   6:  end;

   7:  /

   8:  show errors;

COALESCE(expression1, expression2, …, expressionN), 若果expression1 是null, 就确认expression2, 如果expression2 也是null, 就确认expression3 以此类推, 直到一个不为null 的值. 如果所有的expression都是null, 那么这个函数就返回 null.