Merging multiple rows according to an order - sql

Question

Suppose there are the following rows
| Id | MachineName | WorkerName | MachineState |
|----------------------------------------------|
| 1 | Alpha | Young | RUNNING |
| 1 | Beta | | STOPPED |
| 1 | Gamma | Foo | READY |
| 1 | Zeta | Zatta | |
| 2 | Guu | Niim | RUNNING |
| 2 | Yuu | Jaam | STOPPED |
| 2 | Nuu | | READY |
| 2 | Faah | Siim | |
| 3 | Iem | | RUNNING |
| 3 | Nyt | Fish | READY |
| 3 | Qwe | Siim | |
We want to merge these rows according to following priority :
STOPPED > RUNNING > READY > (null or empty)
If a row has a value for greatest priority, then value from that row should be used (only if it is not null). If it is null, a value from any other row should be used. The rows should be grouped by id
The correct output for the above input is :
| Id | MachineName | WorkerName | MachineState |
|----------------------------------------------|
| 1 | Beta | Foo | STOPPED |
| 2 | Yuu | Jaam | STOPPED |
| 3 | Iem | Fish | RUNNING |
What would be a good sql query to accomplish this? I tried using joins, but it did not work out.

Answer 1

You can view this as a case of the group-wise maximum problem, provided you can obtain a suitable ordering over your MachineState column—e.g. by using a CASE expression:
SELECT a.Id,
COALESCE(a.MachineName, t.MachineName) MachineName,
COALESCE(a.WorkerName , t.WorkerName ) WorkerName,
a.MachineState
FROM myTable a JOIN (
SELECT Id,
MIN(MachineName) AS MachineName,
MIN(WorkerName ) AS WorkerName,
MAX(CASE MachineState
WHEN 'READY' THEN 1
WHEN 'RUNNING' THEN 2
WHEN 'STOPPED' THEN 3
END) AS MachineState
FROM myTable
GROUP BY Id
) t ON t.Id = a.Id AND t.MachineState = CASE a.MachineState
WHEN 'READY' THEN 1
WHEN 'RUNNING' THEN 2
WHEN 'STOPPED' THEN 3
END
See it on sqlfiddle:
| id | machinename | workername | machinestate |
|----|-------------|------------|--------------|
| 1 | Beta | Foo | STOPPED |
| 2 | Yuu | Jaam | STOPPED |
| 3 | Iem | Fish | RUNNING |
You could save yourself the pain of using CASE if MachineState was an ENUM type column (defined in the appropriate order). It so happens in this case that a simple lexicographic ordering over the string value will yield the same result, but that's a coincidence on which you really shouldn't rely as it's bound to slip under the radar when someone tries to maintain this code in the future.

Answer 2

This is a prioritization query. One method uses variables. Another uses union all . . . this works if the states are not repeated for a given id:
select t.*
from table t
where machinestate = 'STOPPED'
union all
select t.*
from table t
where machinestate = 'RUNNING' and
not exists (select 1 from table t2 where t2.id = t.id and t2.machinestate in ('STOPPED'))
union all
select t.*
from table t
where machinestate = 'READY' and
not exists (select 1 from table t2 where t2.id = t.id and t2.machinestate in ('STOPPED', 'RUNNING'));

Answer 3

change MachineState as enum:
`MachineState` enum('READY','RUNNING','STOPPED') DEFAULT NULL
and sql is simple:
select t.id,state.machinename,state.workername,t.mstate from state,(select id,max(MachineState) mstate from state group by Id) t where t.mstate=state.machinestate and t.id=state.id;