sql - Group by alias (Oracle)

Question

How to 'group by' a query using an alias, for example:

select count(*), (select * from....) as alias_column 
from table 
group by alias_column

I get 'alias_column' : INVALID_IDENTIFIER error message. Why? How to group this query?

Answer 1

select
  count(count_col),
  alias_column
from
  (
  select 
    count_col, 
    (select value from....) as alias_column 
  from 
    table
  ) as inline
group by 
  alias_column

Grouping normally works if you repeat the respective expression in the GROUP BY clause. Just mentioning an alias is not possible, because the SELECT step is the last step to happen the execution of a query, grouping happens earlier, when alias names are not yet defined.

To GROUP BY the result of a sub-query, you will have to take a little detour and use an nested query, as indicated above.