在pl/sql中，有三种集合类型：index-by tables(associative arrays,9i之前plsql table),nested table,arrays,它们都可以作为装载集合数据的载体，但有以下区别：
1. Nested table 和arrays 可以作为column,但index-by tables不可以

SQL> create type nested_type is table of varchar2(30);
  2  /

Type created.

SQL> create type varray_type is varray(20) of varchar2(30);
  2  /

Type created.
SQL> create table test(c1 varray_type,c2 nested_type) nested table c2 store as nested_type_tab;

Table created.
SQL> insert into test values(varray_type('aaa','bbb'),nested_type('ccc','ddd'));

1 row created.

2.index-by tables仅用于pl/sql,而Nested tables and arrays可用于pl/sql 和SQL
SQL> declare
  2  type asso_type is table of varchar2(30) index by pls_integer;
  3  lv_asso_type asso_type;
  4  begin
  5  lv_asso_type(1) := 'test';
  6  dbms_output.put_line(lv_asso_type(1));
  7  end;
  8  /
test

PL/SQL procedure successfully completed.

SQL> declare
  2  lv_nested_type nested_type :=nested_type();
  3  lv_varray_type varray_type :=varray_type();
  4
  5  lv_nested_type2 nested_type :=nested_type();
  6  lv_varray_type2 varray_type :=varray_type();
  7  begin
  8
  9  lv_nested_type.extend(100);
 10  lv_nested_type(100) :='test1';
 11  dbms_output.put_line(lv_nested_type(100));
 12
 13  select lv_nested_type
 14  into lv_nested_type2
 15  from dual;
 16
 17  dbms_output.put_line(lv_nested_type2(100));
 18
 19
 20  lv_varray_type.extend;
 21  lv_varray_type(1) :='test2';
 22  dbms_output.put_line(lv_varray_type(1));
 23
 24  select lv_varray_type
 25  into lv_varray_type2
 26  from dual;
 27  dbms_output.put_line(lv_varray_type2(1));
 28
 29  end;
 30  /
test1
test1
test2
test2

PL/SQL procedure successfully completed.