Oracle SQL Error : Unable to execute Subquery

I am trying to execute below Oracle SQL query

select m.*, n.region_building_count from 
(SELECT
  a.BUILDING_ID as "Building Id",
  d.Building_name as "Building Name",
  d.ADDRESS as "Building Address",
  d.REGION as "Region",
  d.Country as "Country",
  d.State as "State",
  d.City as "City"
FROM Company a, LOCATION d
where 1=1
and a.Building_ID like 'BLD-%') m
left JOIN
(select count(building_id) region_building_count, region
 from LOCATION l 
 where l.Building_ID like 'BLD-%' group by region) n
on m.region = n.region

But keep getting error like

Oracle database error 904: ORA-00904: "M"."REGION": invalid identifier 

Any idea what I am missing

Answer

If you use double-quotes when you define an alias like "Region" (which you should never do!), the alias becomes case-sensitive. When you reference m.region, that is not in double-quotes, the column name is automatically converted to upper-case REGION, which does not match Region.

Just don’t use double-quotes around the aliases in the query and this problem will be cured. I didn’t check the rest of the query to see if you will run into additional errors.

Alternatively, you could use m."Region" everywhere (in double quotes). Bad practice!