MYSQL many to many 3 tables query

Question

EDIT. I missed the one main issue I was having. I want to display all the unique 'device_MAC' rows. So I want this query to output 3 rows (as per the original query). The issue I am having is connecting the data table to the remote_node table via dt_short = rn_short where the maximum timestamp for dt_short in the data table.

I am having trouble running a query on 3 tables (2 have many to many relations).

What I am trying to do:

1. Get each distinct rn_IEEE from the remotenodes table with the maximum timestamp (in the example this will get 3 rows with 3 distinct short addresses rn_short )
2. Join with the devicenames table on device_IEEE
3. Get each distinct dt_short from the data table with the maximum timestamp
4. Join dt_short with rn_short from the query above

Now the problem I am running into is that I can do the queries for the above individually, I have even gotten the first 3 of them together into a query but I cannot seem to properly join the last bit of data to get the result that I want.

I have been going in circles trying to solve this. Here is a link to SQL Fiddle which contains all the test data and the query as far as I got it, it does what i want for the first line but from table 'data' after the first line is NULL:

See this SQL fiddle

Answer 1

After going through your requirements and the data, it looks like you just need to change your query to include an INNER JOIN on the data table instead of a LEFT JOIN

See SQL Fiddle with Demo

select rn.*, dn.*, d.*
from remotenodes rn
inner join devicenames dn
  on rn.rn_IEEE = dn.device_IEEE
  and rn.rn_timestamp = (SELECT MAX(rn_timestamp) FROM remotenodes 
                              WHERE rn.rn_IEEE = rn_IEEE 
                              GROUP BY rn_IEEE)
inner join data d
  on rn.rn_short = d.dt_short
  AND d.dt_timestamp = (SELECT MAX(d2.dt_timestamp) AS ts
                        FROM data d2 
                        WHERE d.dt_short = d2.dt_short
                        GROUP BY d2.dt_short)

Answer 2

Try this query, for me its returning one row:

SELECT rn_short, rn_IEEE, device_name
FROM 
(SELECT DISTINCTROW dt_short FROM (SELECT * FROM `data` ORDER BY `dt_timestamp` DESC) as data ) as a
JOIN 
(SELECT rn_IEEE, rn_short, device_name FROM devicenames dn JOIN (SELECT DISTINCTROW rn_IEEE, rn_short FROM (SELECT * FROM `remotenodes` ORDER BY `rn_timestamp` DESC) as remotenodes GROUP BY rn_IEEE) as rn ON dn.device_IEEE = rn.rn_IEEE) as b
ON a.dt_short = b.rn_short

Answer 3

what you have done the query in your SQL fiddle is right.Instead of using left join use inner join so that it will give you the first row

cheers.

Answer 4

Thanks for all your answers everyone. I managed to solve the problem by using views. It's not the most efficient way but I think it will do for now. Here is the SQL Fiddle link:

http://sqlfiddle.com/#!2/4076e/8