I have two tables in MySQL as
+---------+-----------+
| machine | status |
+---------+-----------+
| 40001 | Completed |
| 40001 | Completed |
| 40001 | Completed |
| 40001 | Completed |
| 40001 | Pending |
| 40001 | Pending |
| 40001 | Pending |
| 40001 | Pending |
| 40001 | Pending |
| 40001 | Pending |
+---------+-----------+
And the other one as
+---------+---------+
| machine | packets |
+---------+---------+
| 40001 | 527 |
| 40001 | 1497 |
| 40002 | 1414 |
| 40002 | 2796 |
| 40003 | 392 |
| 40003 | 1663 |
| 40004 | 500 |
| 40004 | 1277 |
+-------+----------+
I want to write a select query which gives me the machine, completion count, pending count and max of packets for that machine. So I tried
SELECT machine,max(packets) AS sync,
sum(if(laststatus='completed', 1, 0)) AS generation,
sum(if(laststatus != 'completed', 1, 0)) AS pending
FROM machine_status
right join machine_packets on machine_packets.machine=machine_status.machine
GROUP BY machine
But I got :
+---------+------+------------+---------+
| machine | sync | generation | pending |
+---------+------+------------+---------+
| 40001 | 1497 | 8 | 2 |
| 40002 | 2796 | 4 | 2 |
| 40003 | 1663 | 6 | 0 |
| 40004 | 1277 | 0 | 2 |
| 40005 | 2755 | 0 | 0 |
| 40006 | 927 | 0 | 0 |
| 40007 | 306 | 0 | 0 |
+---------+------+------------+---------+
As we can see the values are doubled in generation and pending column. Where did I go wrong ?
SELECT machine,sync,
sum(if(laststatus='completed', 1, 0)) AS generation,
sum(if(laststatus != 'completed', 1, 0)) AS pending
FROM machine_status
right join (select machine,
max(packets) AS sync
from machine_packets
group by machine) mp on mp.machine=machine_status.machine
GROUP BY machine
They are doubled because machine_packets has 2 records for each id. To avoid this you can move it in a subquery
A safe way to do this is to use union all
:
select machine, sum(status = 'completed') then generation,
sum(status <> 'completed') then pending,
max(packets) as packets
from ((select machine, status, 0 as packets
from machine_status
) union all
(select machine, 0, packets
from machine_packets
)
) m
group by machine;
This will include rows for all machines in either table.
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