PHP & Mysql - Left Outer Join between two tables - php

Question

I have two tables called 'events' and 'topics' each table can have many comments.
What I need to do is list all the events and topics with the amount of comments for each row. I've managed to return all the topics, which works great but I don't know how I can add the events table to the MySql. The comments and events table fields are listed below. Can anyone help me with this query?
Events:
ID
Event_Name
Comments:
post_id <-- the releated id for either the events or topics table
table <-- The table that the row belongs to so either topics or events
SELECT
t.id, t.title, c.created_at,
IF(ISNULL(c.allComments), 0, c.allComments) AS totalComments
FROM topics AS t
LEFT OUTER JOIN (
SELECT created_at, post_id, COUNT(*) AS allComments
FROM comments
GROUP BY post_id
) AS c ON c.post_id = t.id
ORDER BY tc.created_at DESC, c.allComments DESC

Answer 1

Sounds like events and topics should be the same table.
Still, I think we can do this with a UNION. Events and Topics have the same columns i hope? (Or at least the same important ones?)
(SELECT c.table as event_or_topic, e.*, count(C.table), MAX(C.created_at) as latest_c
FROM events E LEFT JOIN comments C on (C.post_id = E.id)
WHERE C.table = 'Events'
GROUP BY C.post_id)
UNION
(SELECT c.table as event_or_topic, t.id*, count(C.table), MAX(C.created_at) as latest_c
FROM topics T LEFT JOIN comments C on (C.post_id = E.id)
WHERE C.table = 'Topics'
GROUP BY C.post_id)
ORDER BY latest_c
Notice that the ORDER BY applies to the whole UNION, not the individual SELECTs.
The use of LEFT JOIN should allow those rows without Comments to still show. I think the problem is that we have parts of our select dependent on comments (ie - C.table, ordering on last comment, etc). The count should be fine - will just be zero if there are no comments.
You might need to change the SELECT part slightly. I'd like to display C.table so you know whether a row is a topic or event, but im afraid it might screw up the count. Do you need anything from comments besides the count? You use some columns other than post_id and table in your query that you neglected to explain in your question.
You still have columns I don't know what they are, like Comment's zoneTable

Answer 2

Try this:
SELECT
t.id, t.title, c.created_at, COUNT(c.allComments) AS totalComments
FROM topics AS t LEFT JOIN comments c ON t.id=c.post_id
GROUP BY t.id ORDER BY tc.created_at DESC, c.allComments DESC

Answer 3

If I understand your question you have 3 tables:
-Events
-Topics
-Comments
If that is true something like this should extract all the data:
SELECT *
FROM events,topics
LEFT JOIN comments ON post_ID = ID
ORDER BY date DESC
Hope i'm along the right lines!
W.

Answer 4

I've got it working. If anyone knows of a better and an efficient way of doing this, then please let me know:
(SELECT t.id, t.title, tc.dateCreated AS commentDate,
IF(ISNULL(tc.allComments), 0, tc.allComments) AS totalComments,
t.LastActive as dateChanged
FROM Events AS t
LEFT OUTER JOIN (
SELECT MAX(created_at) AS dateCreated, post_id,
COUNT(*) AS allComments
FROM comments
GROUP BY post_id
) AS tc ON tc.post_id = t.id)
UNION
(SELECT t.id, t.title, tc.dateCreated AS commentDate,
IF(ISNULL(tc.allComments), 0, tc.allComments) AS totalComments,
t.LastActive as dateChanged
FROM topics AS t
LEFT OUTER JOIN (
SELECT MAX(created_at) AS dateCreated, post_id,
COUNT(*) AS allComments
FROM comments
GROUP BY post_id
) AS tc ON tc.post_id = t.id)
ORDER BY commentDate DESC, dateChanged DESC, totalComments DESC