Question

Can i use num_daysopen in the second select

SELECT
  DATEDIFF(DATE(NOW()), DATE(dateopened)) AS num_daysopen,
  (28 - DATEDIFF(DATE(NOW()), DATE(dateopened))) as days_left
FROM table

The following does not seem to work, it would be more readable than the above if it did?

SELECT
  DATEDIFF(DATE(NOW()), DATE(dateopened)) AS num_daysopen,
  (28 - num_daysopen) as days_left
FROM table

Answer 1

A column alias isn’t available for use at the same level.
So add a derived table to add a level

SELECT
   num_daysopen,
   (28 - num_daysopen) as days_left
FROM
   (
    SELECT 
       DATEDIFF(DATE(NOW()), DATE(dateopened)) AS num_daysopen
    FROM 
       table
   ) T

See “Problems with Column Aliases” in the MySQL Docs