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Editorial Team
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Asked: 2026-05-28T05:00:30+00:00

I have two tables, message_threads and messages . When returning a results set of

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I have two tables, message_threads and messages. When returning a results set of message_threads, I’m performing a JOIN on the two tables to see whether any message from the sender (to the receiver) within that thread is unread.

Below is my SQL…

SELECT mt.id AS thread_id, m.id AS message_id,
m.is_read, m.from_type, mt.company_id
FROM message_threads AS mt
LEFT JOIN messages AS m
ON m.thread_id = mt.id;

…the full results set from a given query…

+-----------+------------+---------+-----------+------------+
| thread_id | message_id | is_read | from_type | company_id |
+-----------+------------+---------+-----------+------------+
|         1 |          1 | N       | company   |          1 |
|         1 |          9 | N       | company   |          1 |
|         1 |         19 | N       | company   |          1 |
|         2 |          2 | Y       | coder     |          1 |
|         2 |          3 | N       | company   |          1 |
|         2 |          6 | N       | company   |          1 |
|         2 |          8 | N       | company   |          1 |
|         3 |          4 | N       | company   |          1 |
|         6 |         13 | N       | company   |          1 |
|         6 |         14 | N       | coder     |          1 |
|         6 |         15 | N       | company   |          1 |
|         8 |         20 | N       | company   |          1 |
|         8 |         21 | N       | coder     |          1 |
|         4 |          5 | N       | company   |          2 |
|         4 |          7 | N       | company   |          2 |
|         4 |         22 | N       | coder     |          2 |
|         5 |         10 | N       | company   |          8 |
|         5 |         11 | N       | coder     |          8 |
|         5 |         12 | N       | company   |          8 |
|         7 |         16 | N       | company   |         18 |
|         7 |         17 | N       | coder     |         18 |
|         7 |         18 | N       | company   |         18 |
+-----------+------------+---------+-----------+------------+

…and the desired result set:

+-----------+------------+---------+-----------+------------+
| thread_id | message_id | is_read | from_type | company_id |
+-----------+------------+---------+-----------+------------+
|         1 |         19 | N       | company   |          1 |
|         2 |          2 | Y       | coder     |          1 |
|         3 |          4 | N       | company   |          1 |
|         6 |         14 | N       | coder     |          1 |
|         8 |         21 | N       | coder     |          1 |
|         4 |         22 | N       | coder     |          2 |
|         5 |         11 | N       | coder     |          8 |
|         7 |         17 | N       | coder     |         18 |
+-----------+------------+---------+-----------+------------+

How can I perform this query? I’ve tried GROUP BY and DISTINCT, and neither quite do what I want. I also can’t do this using a WHERE clause to filter my data, because I need all the threads regardless of the is_read flag.

Thanks!

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1 Answer

  1. Editorial Team

    There are a lot of ways of doing this. If your looking to see the number of unread messages from a thread you may want to join an inner query.

    Example:

    SELECT 
mt.id AS thread_id, 
mt.company_id,
CASE WHEN m_c.m_unread IS NOT NULL THEN m_c.m_unread ELSE 0 END AS unread
FROM message_threads AS mt
LEFT JOIN 
( 
  SELECT
  thread_id,
  COUNT(*) AS m_unread
  FROM messages
  WHERE
  is_read ='N'
  GROUP BY 1
)m_c
ON mt.thread_id = m_c.thread_id
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