从第二个表中选择相应的值（Mysql）

Question

Struggling with some sql, would appreciate some guidance.

Have two tables logs and sense

logs –

assetid        ts       nodeid  status

1   2017-10-26 14:00:10    73   240
2   2017-10-26 14:00:06    21   160
3   2017-10-26 14:00:04    18   230
4   2017-10-26 14:00:02    19   400
5   2017-10-26 14:00:00    21   190
1   2017-10-26 13:20:08    18   20
2   2017-10-26 13:06:10    20   160
3   2017-10-26 13:03:04    17   230

sense –

status  value
20      5
160     37
190     39
230     56
240     58
400     90

Trying to find the correct syntax to only show the latest record (in datetime) of each assetid and then show the corresponding value from the sense table (based on the matching status in both tables) to produce –

assetid       ts         nodeid     status  value

1   2017-10-26 14:00:10    73        240    58
2   2017-10-26 14:00:06    21        160    37
3   2017-10-26 14:00:04    18        230    56
4   2017-10-26 14:00:02    19        400    90
5   2017-10-26 14:00:00    21        190    39

Have tried –

Select assetid, ts, nodeid, status, value
From
logs
Join sense X on X.status = logs.status
Group by assetid
Order by ts DESC

But this only outputs 1 row (instead of 5)

 assetid          ts      nodeid   status   value

1   2017-10-26 14:00:10    73        240    58

Removing

Join sense X on X.status = logs.status  

of course outputs all records but that is not required.

Thoughts appreciated.

Regards

Active

Answer 1

Actually your query is returning 5 rows, 1 for each id. But it won't return rows with latest ts for each id. You can verify this by clicking on the link for demo. You can compare results of both queries.

To achieve this task,following query will help you:

Select l.assetid, l.ts, logs.nodeid, X.status, X.value
From
logs
inner Join sense X on X.status = logs.status
inner join (select assetid, max(ts) as ts from logs group by assetid) l
on l.assetid = logs.assetid and logs.ts = l.ts
Group by l.assetid
Order by l.ts DESC;

Click here for Demo

EDIT:

If dataype of ts is string then replace max(ts) in above query with:

max(str_to_date(ts,'%d%m%y'))

Feel free to ask any doubts.

Hope it helps!

Answer 2

Use GROUP BY to find minimum for each assetid and then JOIN with the logs and sense

 Select *
 FROM logs l
 JOIN sense s ON s.status = l.status
 JOIN 
 (
  Select assetid, max(ts) maxts
  From logs
  Group by assetid
 ) t ON t.assetid = l.assetid and l.ts = t.maxts

demo

Answer 3

Try this

Select a1.assetid, MAX(a1.ts), a1.nodeid, a1.status, X.value
From
logs a1
inner join sense X on X.status = a1.status
Group by assetid, a1.nodeid, a1.status, X.value
Order by ts DESC

Answer 4

On MY SQL 8.0.2

WITH CTE as 
(
Select A.assetid, A.ts, A.nodeid, A.status, B.value, row_number() over(PARTITION BY A.assetid ORDER BY A.ts DESC) AS rn
from logs as A
inner join sense B ON A.status=B.status
)

SELECT *
FROM CTE
WHERE rn='1';