SQL not recognizing column alias in where clause

Question

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_P         


        

Answer 1

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;