MySQL –左连接在右表中的最后一条记录上

Question

I need a Query using LEFT JOIN where I can find the last record of table 2. For this, I'm looking for the " Id " of Table 1 in Table 2 in the column " channel ".

My current query must be extended for this. I tried to work with several approaches, but did not succeed. I hope you can help me.

https://murrayhopkins.wordpress.com/2008/10/28/mysql-left-join-on-last-or-first-record-in-the-right-table/

Table 1

id  id1  id2 g_id1  g_id2   datum
1    1    2    x     x    timestamp
2    3    1    x     x    timestamp  
3    3    2    x     x    timestamp

Table 2

n_id    channel absender     nachricht            datum
1         1        2        messagetext1    2019-09-22 19:30:31
2         1        1        messagetext2    2019-09-22 19:35:31  
3         2        1        messagetext3    2019-09-22 19:40:31
3         2        3        messagetext4    2019-09-22 19:42:31
4         3        2        messagetext5    2019-09-22 20:40:31

Table 3

id    firmenname  
1    companyname1
2    companyname2    
3    companyname3    

Output

id  id1  id2 g_id1  g_id2   datum      firmenname     firmenname2   nachricht
1    1    2    x     x    timestamp   companyname1   companyname2  messagetext2
2    2    1    x     x    timestamp   companyname2   companyname1  messagetext4
3    3    2    x     x    timestamp   companyname3   companyname2  messagetext5

My current query

SELECT a.*, b.firmenname as firmenname1, c.firmenname as firmenname2 FROM nachrichtensystem a LEFT JOIN spieler b ON a.id1 = b.id LEFT JOIN spieler c ON a.id2 = c.id WHERE id1 = $sp_id OR id2 = $sp_id ORDER BY a.timestamp DESC LIMIT $id, 8

Declaration Table 1 id is the ChannelId. I need the hole Table 1 and the last message from this channel in Table 2. This with the LIMIT and the companyname (firmenname) in Table 3. In my current query is missing only the part with the last message from table 2. The firmenname 1 and 2 comes from table 3 This i need with

ORDER BY timestamp DESC LIMIT $id, 8

Answer 1

You need a subquery that returns the last message for each channel and joined to table1 and then join table3 twice:

select t1.*, t31.firmenname, t32.firmenname firmenname2, t2.nachricht 
from table1 t1 
left join (
  select t.* from table2 t
  where not exists (
    select 1 from table2
    where channel = t.channel and datum > t.datum
  )
) t2 on t2.channel = t1.id
left join table3 t31 on t31.id = t1.id1
left join table3 t32 on t32.id = t1.id2
order by t1.id

See the demo .
Results:

| id  | id1 | id2 | g_id1 | g_id2 | datum     | firmenname   | firmenname2  | nachricht    |
| --- | --- | --- | ----- | ----- | --------- | ------------ | ------------ | ------------ |
| 1   | 1   | 2   | x     | x     | timestamp | companyname1 | companyname2 | messagetext2 |
| 2   | 3   | 1   | x     | x     | timestamp | companyname3 | companyname1 | messagetext4 |
| 3   | 3   | 2   | x     | x     | timestamp | companyname3 | companyname2 | messagetext5 |