What is the default column name given to a TABLE() type with a single column
column_value Pseudocolumn


  When you refer to an XMLTable construct without the COLUMNS clause, or
  when you use the TABLE function to refer to a scalar nested table
  type, the database returns a virtual table with a single column. This
  name of this pseudocolumn is COLUMN_VALUE.


Example:

CREATE OR REPLACE TYPE MISSING_KEYS AS TABLE OF VARCHAR2(256);

SELECT column_value AS missing_key
FROM TABLE (MISSING_KEYS('A','B','C','D'))