COLUMN_VALUE Pseudocolumn

When you refer to an XMLTable construct without the COLUMNS clause, or when you use the TABLE collection expression 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.

In the context of XMLTable, the value returned is of data type XMLType. For example, the following two statements are equivalent, and the output for both shows COLUMN_VALUE as the name of the column being returned:

SELECT *
  FROM XMLTABLE('<a>123</a>');

COLUMN_VALUE
---------------------------------------
<a>123</a>

SELECT COLUMN_VALUE
  FROM (XMLTable('<a>123</a>'));

COLUMN_VALUE
----------------------------------------
<a>123</a>

In the context of a TABLE collection expression, the value returned is the data type of the collection element. The following statements create the two levels of nested tables illustrated in "Creating a Table: Multilevel Collection Example" to show the uses of COLUMN_VALUE in this context:

CREATE TYPE phone AS TABLE OF NUMBER;   
/
CREATE TYPE phone_list AS TABLE OF phone;
/

The next statement uses COLUMN_VALUE to select from the phone type:

SELECT t.COLUMN_VALUE
  FROM TABLE(phone(1,2,3)) t;

COLUMN_VALUE
------------
           1
           2
           3

In a nested type, you can use the COLUMN_VALUE pseudocolumn in both the select list and the TABLE collection expression:

SELECT t.COLUMN_VALUE
  FROM TABLE(phone_list(phone(1,2,3))) p, TABLE(p.COLUMN_VALUE) t;

COLUMN_VALUE
------------
           1
           2
           3

The keyword COLUMN_VALUE is also the name that Oracle Database generates for the scalar value of an inner nested table without a column or attribute name, as shown in the example that follows. In this context, COLUMN_VALUE is not a pseudocolumn, but an actual column name.

CREATE TABLE my_customers (
    cust_id       NUMBER,
    name          VARCHAR2(25),
    phone_numbers phone_list,
    credit_limit  NUMBER)
  NESTED TABLE phone_numbers STORE AS outer_ntab
  (NESTED TABLE COLUMN_VALUE STORE AS inner_ntab);