我想你不能在SQL中做这样的转换。但是在PL / SQL中你可以：

I think you cannot do such casting in SQL. But in PL/SQL you can:

CREATE OR REPLACE TYPE STRARRAY AS TABLE OF VARCHAR2 (255)
/

DECLARE
  tab STRARRAY;
  cnt NUMBER:= 0;
BEGIN
 SELECT COUNT(*)
  INTO cnt
   FROM TABLE(CAST(tab AS strarray));
  dbms_output.put_line(cnt);
END;
/

我认为我错在上面的假设。我没有删除它，因为它仍然是有效的例子。下面的示例使用COLLECT作为table_type的类型来转换现有表列（emp表）：

I think I was wrong in my assumptions above. I did not delete that as it is still valid example. Below example casting existing table column (emp table) with COLLECT as type of table_type:

CREATE OR REPLACE TYPE varchar2_ntt AS TABLE OF VARCHAR2(4000);
/

SELECT deptno
    , CAST(COLLECT(ename) AS varchar2_ntt) AS emps
  FROM   scott.emp
GROUP  BY deptno
/

-- This is dumb but works:

SELECT deptno
     , CAST(COLLECT(ename) AS varchar2_ntt) AS emps
  FROM   scott.emp
 GROUP  BY deptno
 UNION ALL
 SELECT deptno
     , CAST(COLLECT(ename) AS varchar2_ntt) AS emps
   FROM   scott.emp
  GROUP  BY deptno
 /