Question

I often have to write queries with a fairly complex, constructed column that I will aggregate by. For example:

SELECT  
EXTRACT(week FROM to_timestamp("Date Created"/1000)) AS week
...

I know that you cannot use aliases in the GROUP BY clause (this Why doesn't Oracle SQL allow us to use column aliases in GROUP BY clauses? question explains logically why), but is there anything else I can do other than re-doing the column calculation, or am I stuck with this:

SELECT COUNT(*), EXTRACT(week FROM to_timestamp("Date Created"/1000)) AS week
FROM mytable
GROUP BY EXTRACT(week FROM to_timestamp("Date Created"/1000))

Answer 1

Often, I break complexity by using sub-queries.

select count(*), week
from
(
    SELECT EXTRACT(week FROM to_timestamp("Date Created"/1000)) AS week
    FROM mytable
) sel
GROUP BY week

Divide and conquer approach has paid off pretty well so far.

Update

Alternatives to solving this issue:

1. Computed columns (as @gbn stated in his answer).

   • Pros:
     • You can declare a column that’s pretty much used in most queries
     • Some RDBMs allow you to create an index over a computed column (pretty important for performance)
   • Cons:
     • not all RDBMs provide computed columns
     • You might end up declaring a column that’s used in one very specific query (out of the thousands of queries you have in your system). Someday, this query will have changed and the column will just sit there…

2. CTEs