i am using MYSQL and i would like to "SELECT' a table and sort according to a numerical value that i either get it from "column1" or "column2", so:

***if a number from column1 exist, then this number is used to be sorted, otherwise the number from column2 (which always exists) is the number used to be sorted***

this is my current SQL statement:

PHP Code:
FROM table 
WHERE regtime 
ORDER BY regtime ASC 
i think i should use "CASE" but what i can do now is only sort in "column1" first if column1 IS NOT NULL, THEN sort in "column2"

i appreciate your help as this is already one of my last stages on my personal website