SQL not recognizing column alias in where clause

I am only a beginner in SQL, but I’ve come across this annoying error. SQL is having an issue with the WHERE clause of this script:

SELECT
  ITEM_ID, ITEM_PRICE, DISCOUNT_AMOUNT, QUANTITY, 
  (ITEM_PRICE*QUANTITY) AS price_total, 
  (DISCOUNT_AMOUNT*QUANTITY) AS discount_total, 
  ((ITEM_PRICE-DISCOUNT_AMOUNT)*QUANTITY) AS item_total
FROM ORDER_ITEMS
WHERE item_total > 500
ORDER BY item_total;

I am receiving this error:

Error starting at line : 1 in command -
SELECT 
  ITEM_ID, ITEM_PRICE, DISCOUNT_AMOUNT, QUANTITY,  
  (ITEM_PRICE*QUANTITY) AS price_total,  
  (DISCOUNT_AMOUNT*QUANTITY) AS discount_total,  
  ((ITEM_PRICE-DISCOUNT_AMOUNT)*QUANTITY) AS item_total 
FROM ORDER_ITEMS 
WHERE item_total > 500 
ORDER BY item_total DESC;
Error at Command Line : 7 Column : 7
Error report -
SQL Error: ORA-00904: "ITEM_TOTAL": invalid identifier
00904. 00000 -  "%s: invalid identifier"
*Cause:    
*Action:

I have no idea why it has no issue with price_total nor discount_total, but is reporting item_total as invalid. I am trying to first select only the items which have a total greater than 500 when the discount amount is subtracted and it is multiplied by the quantity. Then, I need to sort the results in descending order by item_total

Answer

An alias can be used in a query select list to give a column a different name. You can use the alias in GROUP BY, ORDER BY, or HAVING clauses to refer to the column.

Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined.

So, the following query is illegal:

SQL> SELECT empno AS employee, deptno AS department, sal AS salary
  2  FROM emp
  3  WHERE employee = 7369;
WHERE employee = 7369
      *
ERROR at line 3:
ORA-00904: "EMPLOYEE": invalid identifier


SQL>

The column alias is allowed in:

• GROUP BY
• ORDER BY
• HAVING

You could refer to the column alias in WHERE clause in the following cases:

1. Sub-query
2. Common Table Expression(CTE)

For example,

SQL> SELECT * FROM
  2  (
  3  SELECT empno AS employee, deptno AS department, sal AS salary
  4  FROM emp
  5  )
  6  WHERE employee = 7369;

  EMPLOYEE DEPARTMENT     SALARY
---------- ---------- ----------
      7369         20        800

SQL> WITH DATA AS(
  2  SELECT empno AS employee, deptno AS department, sal AS salary
  3  FROM emp
  4  )
  5  SELECT * FROM DATA
  6  WHERE employee = 7369;

  EMPLOYEE DEPARTMENT     SALARY
---------- ---------- ----------
      7369         20        800

SQL>