Re: Getting the latest unique items

"A.M." <agentm@cmu.edu> writes:
> When I try to run the following query:
> select distinct on(student_gradedmaterial.id) ...        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> ... order by submittime desc;     ^^^^^^^^^^^^^^^^^^^

> I get the following error:

> ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY 
> expressions

Indeed.  You might benefit from reading the DISTINCT ON usage example
given in the SELECT reference page,
http://www.ca.postgresql.org/users-lounge/docs/7.3/postgres/sql-select.html
(see under "Description").  Basically, DISTINCT ON is *only* useful when
designed hand-in-hand with an ORDER BY ...
        regards, tom lane