MySQL: Get latest message from 2 tables that are associated with eachother - mysql

Question

First, thank you so much for your help.
I have 2 tables: a conversation table and a message table, and a third table assoc_message__conversation that associates the messages to a conversation.
I need to get the latest message_id and message sent for each conversation specified, along with the conversation_id it is associated with.
Here is a db-fiddle: https://www.db-fiddle.com/f/kxRQeGUYYgQ7FTwi96hbLp/0
As you can see in this example, there are two conversations with conversation_id of 1 and 2, and there are three messages associated to each conversation. Messages 1, 2, and 3 are associated to conversation 1, and messages 4, 5 and 6 are associated to conversation 2.
I need to be able to specify the conversation_id's in the assoc_message__conversation table (IDs 1 and 2), and retrieve the latest message_id, message and the associated conversation_id sent from the message table for each conversation specified.
So the rows it should pull are:
conversation_id | message_id | message
------------------------------------------------
1 | 3 | "Latest message"
------------------------------------------------
2 | 6 | "Latest message"
------------------------------------------------
Thank you so much for your help!

Answer 1

In older versions of MySQL (< 8.0.2), we can use Derived Tables. In a Derived table, we can get the latest send_datetime value for each conversation_id. Also, it is noteworthy that you can provide your filters for conversation_id in the WHERE clause of this subquery.
We can then use this subquery`s result-set and join back to the main tables appropriately, to get the row corresponding to the latest message in a conversation.
Schema (MySQL v5.7)
View on DB Fiddle
Query #1
SELECT
amc.conversation_id,
m.message_id,
m.message
FROM
assoc_message__conversation AS amc
JOIN message AS m
ON m.message_id = amc.message_id
JOIN
(
SELECT
amc1.conversation_id,
MAX(m1.send_datetime) AS latest_send_datetime
FROM
assoc_message__conversation AS amc1
JOIN message AS m1
ON m1.message_id = amc1.message_id
WHERE amc1.conversation_id IN (1,2) -- Here you provide your input filters
GROUP BY amc1.conversation_id
) AS dt
ON dt.conversation_id = amc.conversation_id AND
dt.latest_send_datetime = m.send_datetime;
Result
| conversation_id | message_id | message |
| --------------- | ---------- | -------------- |
| 1 | 3 | Latest message |
| 2 | 6 | Latest message |
In MySQL 8.0.2 and above, we can use Row_Number() functionality. Within a partition of conversation_id, we will determine Row Number for every message, sorted in descending order of send_datetime. In this subquery, you can provide your filters for conversation_id in the WHERE clause.
We will then use this result-set as a Derived Table, and consider only those rows, where Row Number value is 1 (as it will belong to latest send_datetime).
Schema (MySQL v8.0)
View on DB Fiddle
Query #2
SELECT
dt.conversation_id,
dt.message_id,
dt.message
FROM
(
SELECT
amc.conversation_id,
m.message_id,
m.message,
ROW_NUMBER() OVER (PARTITION BY amc.conversation_id
ORDER BY m.send_datetime DESC) AS row_no
FROM
assoc_message__conversation AS amc
JOIN message AS m
ON m.message_id = amc.message_id
WHERE amc.conversation_id IN (1,2) -- Here you provide your input filters
) AS dt
WHERE dt.row_no = 1;
Result
| conversation_id | message_id | message |
| --------------- | ---------- | -------------- |
| 1 | 3 | Latest message |
| 2 | 6 | Latest message |

Answer 2

Assuming that amc_id increments for each new message, I would recommend a correlated subquery in the where clause:
select amc.*, m.message
from message m join
assoc_message__conversation amc
on amc.message_id = m.message_id
where amc.amc_id = (select max(amc.amc_id)
from assoc_message__conversation amc2
where amc2.conversation_id = amc.conversation_id
);
If you actually need to use send_datetime, then an additional join is necessary:
where m.send_datetime = (select max(m2.send_datetime)
from message m2 join
assoc_message__conversation amc2
on amc2.message_id = m2.message_id
where amc2.conversation_id = amc.conversation_id
)