MySQL Two databases, order by one column in one case, order by another column in different case

Question

I have two tables. First table looks like this:

Table: Records

    rid  |   user id   |     title     |    whenadded    | public
--------------------------------------------------------------------
    1        212           Example        2012-06-28         1
    2        217           Test Rec       2012-07-05         1
    3        212           Another        2012-07-02         1
    4        212           Unlisted       2012-05-02         0
    5        217           Success        2012-04-08         1
    6        238           Always         2012-04-18         1

Table: Likes

    id  |   user id   |     rid     |    whenliked
------------------------------------------------------
    1        212            2            2012-07-06
    2        205            1            2012-06-30
    3        212            5            2012-07-04

In the 'Records' table, 'rid' is set as the primary index. In the 'Likes' table, id is set as the primary index.

I'm using PHP. PHP will provide a value to MySQL to use as reference. I'd like to have a single MySQL query that will do the following:

Pseudo Code:

$userid = 212;
$SQL = 'SELECT DISTINCT records.* 
        FROM records,likes 
        WHERE (records.userid = ' . $userid . 
          ' AND records.public = 1) 
            OR (records.id = likes.rid AND likes.userid = ' . $userid . 
          ' AND records.public = 1) 
        ORDER BY likes.whenliked DESC, records.whenadded DESC 
        LIMIT 50;';

Look at the $SQL query I just provided above. That's my attempt at developing the query I wanted, but it didn't achieve what I was looking for; it came pretty damn close, but it was still ordering incorrectly. The query was developed as best as I could first by myself, then based on what I found by searching for a solution on StackFlow and elsewhere on Google.

Those are the conditions I'm trying to order it by:

1. All records selected must be public (records.public = 1).
2. If the record belongs to the user (in this example, 212), then order the records by records.whenadded.
3. If the record doesn't belong to the user, but it's a record the user liked, then order the records by likes.whenliked.
4. Dates would be ordered from newest to oldest.

The end result from the query returned would look like this (keep in mind, whenliked is NOT in the return data, it's just there for reference so you see how it's ordered):

    rid  |   user id   |     title     |    whenadded    | public   |  whenliked {not incl}
------------------------------------------------------------------------------------
    2        217           Test Rec       2012-07-05         1        2012-07-06
    3        212           Another        2012-07-02         1
    5        217           Success        2012-04-08         1        2012-06-30
    1        212           Example        2012-06-28         1

Hope that makes sense. Feel free to ask questions, I'll clarify the best I can.

Thanks in advance for your time, your consideration, and for reading this. Even if there is no response or no solution is found, your time is still very much appreciated! :)

Answer 1

The field you are ordering by doesn't have to be an existing field in the database. You can also use a field you define in your select:

SELECT IF(records.userid = ' . $userid . ', records.whenadded, likes.whenliked) as date 

Now you can use it in the order by part:

ORDER BY date DESC

From the MySQL Manual:

A select_expr can be given an alias using AS alias_name. The alias is used as the expression's column name and can be used in GROUP BY, ORDER BY, or HAVING clauses