Question

I was hoping someone can help me out here.

I am trying to write a query in mysql and i am having a really hard time getting the syntax right. Let me explain what I am trying to do.

I have a table that houses measurement data for construction and each entry has a location_id, room_id, measurement_value, floor_num, measurement_date. I am trying to write a query to get the latest measurement data from each floor (there are only two floors). so I was thinking something like this but I can’t get it to work.

 select bottom_floor.value as bottom_value, top_floor.value as top_value
 from 
 (select measurement_value, measurement_date, floor_num, room_id from rom_measurements where location_id = '1' and floor_num='1' order by measurement_date DESC limit 1) as bottom_floor
 Join 
 (select measurement_value, measurement_date, floor_num, room_id from rom_measurements where location_id = '1' and floor_num='2' order by measurement_date DESC limit 1) as top_floor

So I thought that this would give me back the two values. I have seen that if one of the two subqueries returns an empty set it doesn’t work and I am also pretty sure I have done other things wrong here.

Can someone recommend a better way to accomplish this?

Thanks!

Answer 1

Total square for all rooms on the floor:

select square, measurement_date, floor_num
from room_measurements rm
join (
  select location_id, floor_num, max(measurement_date) as date, sum(measurement_value) as square
  from room_measurements 
  group by location_id, floor_num    
) rm2 
ON rm.measurement_date=rm2.date AND rm.location_id=rm2.location_id AND rm.floor_num=rm2.floor_num
WHERE location_id = '1'

Square for each room on the floor:

select measurement_value, measurement_date, floor_num, room_id
from room_measurements rm
join (
  select location_id, floor_num, room_id, max(measurement_date) as date
  from room_measurements 
  group by location_id, floor_num, room_id    
) rm2 
ON rm.measurement_date=rm2.date AND rm.location_id=rm2.location_id AND rm.floor_num=rm2.floor_num AND rm.room_id = rm2.room_id
WHERE location_id = '1'