Q:

How To Reference a Column that has been concatenated in SQL

How do you reference a column in a table that has been concatenated? I am trying to reference the 'UniqueID' column in a join, but all the ways that I have tried it throw the ORA-00904 error saying "T2.UNIQUE ID:Invalid identifier".  
 create table cdm_user.uniquesubjectIDDEW as (

 select distinct concat (site,screening_no) "UniqueID" , visit, site, Screening_no
from databrowser.v_data_entry_workflow
where study = '3508'

 );

 commit;

Select *
from cdm_user.uniquesubjectIDDEW t1
left join cdm_user.uniquesubjectIDDEW t2
on t1.UniqueID = t2.UniqueID 
and t2.visit = 'Screening'
Where t1.visit = 'Week_52'
and t2.visit is null

Any help is much appreciated as I am new to SQL. 

A:

Unless quoted, identifiers such as table and columns names will be mapped to upper case. So your select will be interpreted as needing a column name UNIQUEID but you created the column name as "UniqueId" with quotes so it doesn't match.
You'll need to either unquote the name when you create the table or quote it in all queries.
Generally it is better not to use quoted, case-sensitive column names, which is why a lot of databases use underscores in table/column names as word separators rather than some variant of camel case.

