题目：
       我执行了这些语句:

CREATE OR REPLACE TYPE plch_numbers_nt IS TABLE OF NUMBER;
/

CREATE OR REPLACE TYPE plch_numbers_vat IS VARRAY (10) OF NUMBER;
/

CREATE OR REPLACE PACKAGE plch_pkg
IS
   TYPE plch_numbers_nt IS TABLE OF NUMBER;
END;
/

CREATE OR REPLACE TYPE plch_numbers_aat IS TABLE OF NUMBER
INDEX BY PLS_INTEGER;
/

       然后我写了如下的不完整代码块：

DECLARE
   l_count     PLS_INTEGER;
   l_numbers   /*TYPE*/ := /*TYPE*/ (1, 2, 3, 4, 5);
BEGIN
   SELECT COUNT (COLUMN_VALUE)
     INTO l_count
     FROM TABLE (l_numbers);

   DBMS_OUTPUT.put_line (l_count);
END;
/

       哪些选项可用来取代 /*TYPE*/ 注释，从而使得代码块执行之后会显示"5"?

(A)
plch_numbers_nt

(B)
plch_numbers_vat

(C)
plch_pkg.plch_numbers_nt

(D)
plch_numbers_aat

       答案：
    
       A: plch_numbers_nt是在SCHEMA级别定义的嵌套表类型(NESTED TABLE)所以可被SQL引擎识别并用TABLE()函数转换为表
       B: plch_numbers_vat是在SCHEMA级别定义的可变数组类型(VARRAY)所以可被SQL引擎识别并用TABLE()函数转换为表
       C: plch_pkg.plch_numbers_nt是在包级别定义的，无法被SQL引擎识别
       D: 关联数组只能在PLSQL中使用，实际上plch_numbers_aat不能成功创建，会报错：

SQL> CREATE OR REPLACE TYPE plch_numbers_aat IS TABLE OF NUMBER
  2   INDEX BY PLS_INTEGER;
  3   /

Warning: Type created with compilation errors.

Elapsed: 00:00:04.76
SQL> SHOW ERR
Errors for TYPE PLCH_NUMBERS_AAT:

LINE/COL ERROR
-------- -----------------------------------------------------------------
0/0      PL/SQL: Compilation unit analysis terminated
1/26     PLS-00355: use of pl/sql table not allowed in this context