SQL not recognizing column alias in where clause

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>

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;