"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