Mysql - select and order by two price columns as one - php

Question

I am facing one problem with selecting data from mysql and I cant figure it out. Try to do research but without luck.
I have table with 4 columns:
id | name | price | discount_price
1 | Test 1 | 150 | 50
4 | test 2 | 130 | 300
2 | test 3 | 200 | 0
3 | test 4 | 130 | 10
4 | test 5 | 80 | 0
And I need to select data and order them by price and if discount_price is not "ZERO", than order by price_discount. Issue is, that now it is not working as I expected. It shows results as first column is sorted and in the end is second column sorted. Depends if I choose ASC or DESC.
I need to achieve something like merge this two columns into one and than sort. Like this:
id | name | price | discount_price
3 | test 4 | 130 | 10*
4 | test 5 | 80* | 0
1 | Test 1 | 150 | 50*
2 | test 3 | 200* | 0
4 | test 2 | 130 | 300*
Is it possible in mysql? Or should sort this afterwards in php ? Thank you for helping.

Answer 1

Try this (the field definitions in the table creation are pseudocode):
CREATE TEMPORARY TABLE sort (id INT, name VARCHAR, INT price, INT discount_price, INT actual_price) ;
INSERT INTO sort (id, name, price, discount_price)
SELECT id, name, price, discount_price
FROM original_table ;
UPDATE sort SET actual_price = price WHERE discount_price = 0 ;
UPDATE sort SET actual_price = discount_price WHERE discount_price != 0 ;
SELECT id, name, price, discount_price FROM sort ORDER BY actual_price ;