离开联接或联盟的MySQL查询？

Question

I'm struggling with a query, I've tried UNION but it needs the same number of column results so it's no good, here's what I'm trying to do:

I'm trying to select the latest rows from table actions:

 action_id INT  |  media_id INT |   badge_id INT  | performed_at TS
     ______________________________________________________________
        1       |        365    |     NULL        | timestamp
        2       |       NULL    |        26       | timestamp
        3       |       62      |       NULL      | timestamp

If media_id isn't NULL, I want to get corresponding row from the table title and if badge_id isn't NULL I want to get the corresponding row from the table badge.

So I tried this (but it doesn't work because title and badges do not have the same number of columns):

 (SELECT * FROM actions, actions_type, a3m_account, title 
            WHERE actions.type_id = actions_type.type_id 
            AND actions.performed_by_user_id = a3m_account.id 
            AND actions.media_id = title.media_id)

            . ' UNION '

            (SELECT * FROM actions, actions_type, a3m_account, badges 
            WHERE actions.type_id = actions_type.type_id
            AND actions.performed_by_user_id = a3m_account.id 
            AND actions.badge_id = badges.badge_id) 


            . 'ORDER BY performed_at DESC '
            . 'LIMIT 10';

Edit: I'm basically creating a twitter style feed with all the actions that were taken on the site. Some of those actions are related to a media (table title) and others are related to a badge (table badge). I want to get those last actions and their related data (the issue being that the related data has to come from different tables which have different columns).

How can I do this?

Answer 1

Since UNION only works on tables with the same schema, the query that you have to write has the following structure (you have not specified the fields you want):

 (SELECT <1st field list> 
 FROM actions, actions_type, a3m_account, title
 WHERE actions.type_id = actions_type.type_id 
 AND actions.performed_by_user_id = a3m_account.id 
 AND actions.media_id = title.media_id)

 UNION

 (SELECT <2nd fields list>
 FROM actions, actions_type, a3m_account
 WHERE actions.type_id = actions_type.type_id
 AND actions.performed_by_user_id = a3m_account.id 
 AND actions.badge_id = badges.badge_id) 

In <1st fields list> and <2nd fields list>, you must have the same number of fields and same type in both field lists. If needed, you can use the AS clause so that you have same field names in both lists.

Example:

SELECT actions.action_id, actions.media_id AS additional_info
FROM ...
UNION 
SELECT actions.action_id, actions.badge_id AS additional_info
FROM ...

Answer 2

In order of importance:

1. Rather than select * from all tables, select * from the common tables and specifically select columns from title and badge such that the number of columns selected from those tables is the same and have the same type
2. And where media_id is not null to the first query and where badge_id is not null to the second query. Note that if both id's are present, you'll get two rows for the item
3. Remove the brackets from sound your selects (so they are not inner queries, but just part of the one query
4. Use proper join syntax (ie use the join keyword)