MySQL Query Two Tables and Max Timestamp - mysql

Question

I have two tables that look something like this:
TABLE_conversations:
+-----------------+----------+----------------+------------+---------------------+--------+
| CONVERSATION_ID | QUEUE_ID | CONTACT_NUMBER | CONTACT_ID | DATE_CREATED | STATUS |
+-----------------+----------+----------------+------------+---------------------+--------+
| 1 | 1 | 15551112222 | 9000001 | 2014-09-12 00:28:24 | ACTIVE |
| 2 | 1 | 15553334444 | 9000002 | 2014-09-12 00:32:08 | ACTIVE |
+-----------------+----------+----------------+------------+---------------------+--------+
TABLE_messages:
+------------+-----------------+-------------+-------------+-----------+---------+---------------------+--------+-----------------------------------------------------------------------------------------------------------------+--------+
| MESSAGE_ID | CONVERSATION_ID | FROM_NUMBER | TO_NUMBER | DIRECTION | SENDER | TIMESTAMP | VIEWED | MESSAGE | STATUS |
+------------+-----------------+-------------+-------------+-----------+---------+---------------------+--------+-----------------------------------------------------------------------------------------------------------------+--------+
| 1 | 1 | 15551112222 | 17021112222 | IN | 9000001 | 2014-09-12 00:30:11 | 1 | Hello! Is this working? | ACTIVE |
| 2 | 1 | 17021112222 | 15551112222 | OUT | 8000001 | 2014-09-12 00:31:05 | 1 | Good evening! Of course! | ACTIVE |
| 3 | 1 | 15551112222 | 17021112222 | IN | 9000001 | 2014-09-12 00:31:27 | 1 | Perfect. Thank you! | ACTIVE |
| 4 | 1 | 17021112222 | 15553334444 | OUT | 8000002 | 2014-09-12 00:32:52 | 1 | Ticket 11251 is ready for pickup. | ACTIVE |
+------------+-----------------+-------------+-------------+-----------+---------+---------------------+--------+-----------------------------------------------------------------------------------------------------------------+--------+
I'm trying to run a query to select the CONVERSATION_ID, CONTACT_NUMBER, CONTACT_ID and most recent TIMESTAMP and grouping by phone number:
SELECT TABLE_conversations.CONVERSATION_ID, TABLE_conversations.CONTACT_NUMBER,
TABLE_conversations.CONTACT_ID, MAX(TABLE_messages.TIMESTAMP)
FROM TABLE_conversations, TABLE_messages
WHERE TABLE_conversations.STATUS='ACTIVE'
AND TABLE_messages.STATUS='ACTIVE'
GROUP BY CONTACT_NUMBER
ORDER BY TABLE_messages.TIMESTAMP;
The output I'm getting is below:
+-----------------+----------------+------------+-------------------------------+
| CONVERSATION_ID | CONTACT_NUMBER | CONTACT_ID | MAX(TABLE_messages.TIMESTAMP) |
+-----------------+----------------+------------+-------------------------------+
| 1 | 15551112222 | 9000001 | 2014-09-12 00:32:52 |
| 2 | 15553334444 | 9000002 | 2014-09-12 00:32:52 |
+-----------------+----------------+------------+-------------------------------+
I'm getting the same TIMESTAMP for both. The result I want is 2014-09-12 00:31:27 for 15551112222 and 2014-09-12 00:32:52 for 15553334444.
Really appreciate any help!

Answer 1

You're missing join conditions between the tables, so you're getting a full cross-product. So every conversation is being joined with every message, not just the messages from that conversation.
SELECT TABLE_conversations.CONVERSATION_ID, TABLE_conversations.CONTACT_NUMBER,
TABLE_conversations.CONTACT_ID, MAX(TABLE_messages.TIMESTAMP)
FROM TABLE_conversations
JOIN TABLE_messages ON TABLE_conversations.conversation_id = TABLE_messages.conversation_id
WHERE TABLE_conversations.STATUS='ACTIVE'
AND TABLE_messages.STATUS='ACTIVE'
GROUP BY CONTACT_NUMBER
ORDER BY TABLE_messages.TIMESTAMP;

Answer 2

Your sql cross join all rows from two tables, so the max timestamp for any group is the same.
SELECT TABLE_conversations.CONVERSATION_ID,
TABLE_conversations.CONTACT_NUMBER,
TABLE_conversations.CONTACT_ID,
MAX(TABLE_messages.TIMESTAMP)
FROM TABLE_conversations
JOIN TABLE_messages
ON TABLE_conversations.CONVERSATION_ID = TABLE_messages.CONVERSATION_ID
WHERE TABLE_conversations.STATUS='ACTIVE'
AND TABLE_messages.STATUS='ACTIVE'
GROUP BY CONTACT_NUMBER
Suggest you to get rid of the group by and aggregation function to see what's different between full cross join and inner join. Such as:
SELECT TABLE_conversations.CONVERSATION_ID,
TABLE_conversations.CONTACT_NUMBER,
TABLE_conversations.CONTACT_ID,
TABLE_messages.TIMESTAMP
FROM TABLE_conversations
JOIN TABLE_messages
ON TABLE_conversations.CONVERSATION_ID = TABLE_messages.CONVERSATION_ID
--without on clause above, comes to the full cross join
WHERE TABLE_conversations.STATUS='ACTIVE'
AND TABLE_messages.STATUS='ACTIVE'
ORDER BY TABLE_messages.TIMESTAMP;