Select column from another table if it matches a content - mysql

Question

I've a SELECT which checks a status of active alarms (icinga).
This select joins different tables and until here all ok.
On the result I've as value/column an object_id as well. I would like to add a column to that select that could be empty or not, because, searching that 'object_id' on a different table, I could get a value or not. This accessory table is structured having: object_id, varname, varvalue.
So, i.e., my SELECT returns those values:
`name`, `object_id`, `status`
`Hello`, `123456`, `OK`
I would add the column City that should compared to a table having:
`object_id`, `varname`, `varvalue`
`123456`, `city`, `Rome`
`123456`, `lake`, `Garda`
`789789`, `city`, `Milano`
So that if the second table has object_id = 123456 AND city = Rome the result should be:
`name`, `object_id`, `status`, `city`
`Hello`, `123456`, `OK`, `Rome`
Otherwise the result should be:
`Hello`, `123456`, `OK`, `UNKNOWN`
How to do that?
Hope I've explained it well :-)
Thanks!
* EDIT *
It's better I explain with real example. My query actually is the following:
select icinga_objects.object_id, icinga_objects.name1 as host_name, icinga_objects.name2 as ServiceName, "service" as Type, icinga_servicestatus.last_check as LastCheckTime, icinga_servicestatus.last_hard_state_change as LastStateChange, TIMEDIFF(now(), icinga_servicestatus.last_hard_state_change) AS SinceTime,
CASE
WHEN icinga_servicestatus.current_state = 0 THEN '0'
WHEN icinga_servicestatus.current_state = 1 THEN '2'
WHEN icinga_servicestatus.current_state = 2 THEN '3'
ELSE '3'
END AS state
FROM icinga_objects, icinga_servicestatus, icinga_services WHERE icinga_servicestatus.service_object_id IN
(SELECT service_object_id FROM icinga_services WHERE icinga_services.host_object_id IN
(SELECT host_object_id FROM icinga_hostgroup_members WHERE hostgroup_id IN
(SELECT hostgroup_id FROM icinga_hostgroups WHERE alias = 'MY-HOSTGROUP-TO-FILTER')
)
)
AND icinga_servicestatus.service_object_id NOT IN
(SELECT service_object_id FROM icinga_services WHERE icinga_services.service_object_id IN (
SELECT object_id FROM icinga_objects WHERE icinga_objects.is_active = 1 AND icinga_objects.object_id IN
(SELECT object_id FROM icinga_customvariables WHERE varvalue = '8x5')
)
)
AND icinga_servicestatus.last_check > NOW() - INTERVAL 3 HOUR
AND icinga_servicestatus.state_type = 1
AND icinga_servicestatus.scheduled_downtime_depth = 0
AND icinga_objects.object_id = icinga_services.service_object_id
AND icinga_servicestatus.service_object_id = icinga_services.service_object_id
AND icinga_servicestatus.current_state = 2
AND icinga_servicestatus.problem_has_been_acknowledged = 0
This gives me as result, in example:
`object_id`, `host_name`, `ServiceName`, `Type`, `LastCheckTime`, `LastStateChange`, `SinceTime`, `State`
`123456`, `myHostName`, `myServiceName`, `service`, `2020-04-29 17:19:21`, `2020-04-28 14:50:27`, `26:32:51`, `3`
Here I would like to add the column.
So, now if I search object_id into icinga_customvariables I could find entries, or not. In Example, searching object_id = 123456 I have 4 records, but ONLY one having varname = NAME_IM_SEARCHING and so I need to add to the above result the corresponding of varvalue searching icinga_customvariables.object_id = '123456' AND varname = NAME_IM_SEARCHING. IF there are NO results, then the added cloumn should be UNKNOWN, otherwise the added column should be = icinga_customvariables.varvalue.
How to add it? :-)

Answer 1

You can place your query into a "table expression" so it becomes simpler to join it to the other_table. For example:
select
q.*,
coalesce(o.varvalue, 'UNKNOWN') as city
from (
-- your existing query here
) q
left join other_table o on o.object_id = q.object_id and o.varname = 'city'
EDIT: Joining multiple times
As requested if you need to extract more city names using another column, or if you want to extract against another table altogether, you can add an extra LEFT JOIN. For example:
select
q.*,
coalesce(o.varvalue, 'UNKNOWN') as city,
coalesce(o2.varvalue, 'UNKNOWN') as lake
from (
-- your existing query here
) q
left join other_table o on o.object_id = q.object_id and o.varname = 'city'
left join other_table o2 on o.object_id = q.object_id and o2.varname = 'lake'