How can I sort by a different column if first choice is null

Question

I have a movie database. I want to sort by released date but when released date is missing (null or empty) i want to sort by dateAdded instead

SELECT 
    movie.ID,
    Title,
    released,
    dateAdded
FROM
    movie
WHERE
    dateAdded is not null 
    and (released <= CURDATE() or released is null or released ='')
ORDER BY released DESC
LIMIT 50

Answer 1

Usually you do it like this:

ORDER BY IFNULL(released, dateAdded)

That's somewhat abusive since it can't be indexed properly but will work on relatively large amounts of data without bogging down too badly.

Answer 2

最好使用Standard SQL的COALESCE而不是专有的ISNULL ：

ORDER BY COALESCE(released, dateAdded) DESC