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

Answer 1

To use an alias in Oracle you need to ensure that the alias has been defined by your query at the point at which the alias is being used.

The most straightforward way to do this is to simply treat the original query as a subquery -- in this case,

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

becomes

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

I can't speak to the performance implications, but it's very quick to write if you're trying to re-use an alias in your query - throw everything in parentheses and jump up a level...