ORA-00904 : invalid identifier [duplicate] - sql

Question

This question already has answers here:
Using an Alias in a WHERE clause
(5 answers)
Closed 8 months ago.
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 1

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:
Sub-query
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>

Answer 2

Starting from Oracle 12c you could use CROSS APPLY to define expression and then you could refer to them in WHERE clause:
SELECT
o.ITEM_ID, o.ITEM_PRICE, o.DISCOUNT_AMOUNT, o.QUANTITY,
s.price_total, s.discount_total, s.item_total
FROM ORDER_ITEMS o
CROSS APPLY (SELECT ITEM_PRICE*QUANTITY AS price_total,
DISCOUNT_AMOUNT*QUANTITY AS discount_total,
(ITEM_PRICE-DISCOUNT_AMOUNT)*QUANTITY AS item_total FROM dual) s
WHERE s.item_total > 500
ORDER BY s.item_total;

Answer 3

You cannot use the column name which is used as alias one in the query
Reason:
The query will first checks for runtime at that time the column name "item_total" is not found in the table "ORDER_ITEMS" because it was give as alias which is not stored in anywhere and you are assigning that column in desired output only
Alternate:
If you want to use that type go with sub queries it's performance is not good but it is one of the alternate way
SELECT * FROM
(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) as tbl
WHERE tbl.item_total > 500
ORDER BY tbl.item_total;