Question

In the SQL statement below the results are returned out of order. If they were in ascending or descending order it would make sense, but it seems there is no order here.

Any insights as to why the ORDER BY function isn’t sorting by the ‘date’ alias would be very much appreciated.

SQL Statement:

SELECT id, date, type 
    FROM (SELECT resume_id AS id, DATE_FORMAT( date_mod, '%M %e, %Y' ) AS date, 'resume' AS TYPE 
              FROM resumes 
              WHERE user_id = '$user_id' 
          UNION ALL 
          SELECT profile_id, DATE_FORMAT( date_mod, '%M %e, %Y' ) AS date, 'profile' 
              FROM profiles 
              WHERE user_id = '$user_id' 
          ORDER BY date DESC LIMIT 5) AS d1 
    ORDER BY date

Results:

Resume was updated on February 14, 2012
Resume was updated on February 15, 2012
Resume was updated on February 15, 2012
Resume was updated on February 9, 2012
Profile was updated on February 9, 2012

Answer 1

It’s sorting them as strings because you’ve converted the dates to strings using DATE_FORMAT (note that as a string “February 15…” is lower than “February 9…” because 1 comes before 9 in the “alphabet”). The solution is to sort by the real date in date_mod. You could do it directly by just adding in date_mod to the selects and changing the order, like this:

SELECT id, date, date_mod, type
FROM (
  SELECT resume_id AS id,
    DATE_FORMAT(date_mod, '%M %e, %Y') AS date,
    date_mod,
    'resume' AS type
  FROM resumes
  WHERE user_id = '$user_id'
  UNION ALL
  SELECT profile_id AS id,
    DATE_FORMAT(date_mod, '%M %e, %Y') AS date,
    date_mod,
    'profile' AS type
  FROM profiles
  WHERE user_id = '$user_id'
  ORDER BY date_mod DESC
  LIMIT 5
) AS d1
ORDER BY date_mod

But even better would be to simplify it by selecting only date_mod in the sub-query (i.e. no formatted version) and doing the DATE_FORMAT last in the outer query:

SELECT id, DATE_FORMAT(date_mod, '%M %e, %Y') AS date, type
FROM (
  SELECT resume_id AS id, date_mod, 'resume' AS type
  FROM resumes
  WHERE user_id = '$user_id'
  UNION ALL
  SELECT profile_id AS id, date_mod, 'profile' AS type
  FROM profiles
  WHERE user_id = '$user_id'
  ORDER BY date_mod DESC
  LIMIT 5
) AS d1
ORDER BY date_mod