sql - PostgreSQL GROUP BY LOWER() not working -

i trying use group by in postgresql 9.4.1 , not having success hoped.

there several folks on the web claim should work, can't same results. want case-insensitive group by every time add lower() complains with:

error: column "people.fn" must appear in group clause or used in aggregate function

create database test; create table people (id integer, fn text, ln text); /* id, firstname, lastname */  insert people (id, fn, ln) values(1,'mike','f'); insert people (id, fn, ln) values(2,'adam','b'); insert people (id, fn, ln) values(3,'bill','b'); insert people (id, fn, ln) values(4,'bill','r'); insert people (id, fn, ln) values(5,'mike','d'); insert people (id, fn, ln) values(6,'mike','c'); insert people (id, fn, ln) values(7,'mike','t');  select fn people group lower(fn);  /* not run */ select fn people group fn;  /* runs, not want */ 

here's get:

adam mike mike bill bill 

here's want:

mike adam bill 

obviously, there's i'm missing. , no, can't sanitize data put database. should read understand this?

generally, if want select in aggregate query, have group "something". in case, can results want selecting lower(fn):

select lower(fn) people group lower(fn) 

luckily, postgresql allows group alias, don't have repeat lower(fn) twice:

select lower(fn) lfn people group lfn 

sql fiddle demo

as @okaram mentioned in comments, if don't need other aggrgation on table, you'd better use distinct:

select distinct lower(fn) people