Create a nested table and insert data only in the inner table - oracle

Question

I have this table: employees(id, firstname, lastname, salary) and I added another column that will store all the past salaries of every employee.
CREATE OR REPLACE TYPE salary_list AS TABLE OF NUMBER;
/
ALTER TABLE employees
ADD (salary_history salary_list)
NESTED TABLE salary_history STORE AS salary_history_tab;
Now, the table is employees(id, firstname, lastname, salary, salary_history) and salary_history is null. When I modify the salaray value, how do I insert the current salary value into the inner table salary_history? I have tried:
INSERT INTO
TABLE(SELECT salary_history FROM employees WHERE id=1)
VALUES(1500);
And I get the following error:
ORA-22908: reference to NULL table value
Yes, the nested table salary_history is null because I just created it. What am I doing wrong? What is the correct way to insert values(append to the existing data) into salary_history identifying the employee by id?
UPDATE:
added nvl(,()) like so:
INSERT INTO
TABLE(SELECT nvl(salary_history,salary_list()) FROM employees WHERE id=1)
VALUES(1000);
The error that I get now is:
ORA-25015: cannot perform DML on this nested table view column

Answer 1

You can use COALESCE( salary_history, salary_list() ) MULTISET UNION ALL salary_list( :your_new_value ) to append the new value to the old list (or create a new list if it does not exist).
Oracle 11g R2 Schema Setup:
CREATE OR REPLACE TYPE salary_list AS TABLE OF NUMBER;
/
CREATE TABLE employees(
id NUMBER,
salary_history salary_list
) NESTED TABLE salary_history STORE AS salary_history_tab
/
INSERT INTO employees VALUES ( 1, NULL )
/
Query 1:
UPDATE employees
SET salary_history = COALESCE( salary_history, salary_list() )
MULTISET UNION ALL salary_list( 500 )
WHERE id = 1
SELECT * FROM employees
Results:
| ID | SALARY_HISTORY |
|----|----------------|
| 1 | 500 |
Query 2:
UPDATE employees
SET salary_history = COALESCE( salary_history, salary_list() )
MULTISET UNION ALL salary_list( 700 )
WHERE id = 1
SELECT * FROM employees
Results:
| ID | SALARY_HISTORY |
|----|----------------|
| 1 | 500,700 |
Query 3:
UPDATE employees
SET salary_history = COALESCE( salary_history, salary_list() )
MULTISET UNION ALL salary_list( 500 )
WHERE id = 1
SELECT * FROM employees
Results:
| ID | SALARY_HISTORY |
|----|----------------|
| 1 | 500,700,500 |

Answer 2

The column is currently null, rather than an empty table; you can create an empty nested table to start with:
UPDATE employees
SET salary_history = NEW salary_list()
WHERE id = 1;
1 row updated.
And your first statement will then work:
INSERT INTO
TABLE(SELECT salary_history FROM employees WHERE id=1)
VALUES(1500);
1 row inserted.
You can see the new value:
SELECT * FROM employees;
ID FIRSTNAME LASTNAME SALARY SALARY_HISTORY
---------- ---------- ---------- ---------- ------------------------------
1 Joe BLoggs 1234 SALARY_LIST(1500)