PostgreSQL GROUP BY LOWER() not working

Question

I am trying to use GROUP BY in PostgreSQL 9.4.1 and not having as much success as I hoped.

There are several folks on the web that claim this should work, but I can't get the same results. All I want is a case-insensitive GROUP BY but every time I add LOWER() it complains with:

ERROR: column "people.fn" must appear in the GROUP BY clause or be used in an aggregate function

CREATE DATABASE TEST;
CREATE TABLE people (id INTEGER, fn TEXT, ln TEXT); /* ID, firstname, lastname */

INSERT INTO people (id, fn, ln) VALUES(1,'Mike','f');
INSERT INTO people (id, fn, ln) VALUES(2,'adam','b');
INSERT INTO people (id, fn, ln) VALUES(3,'bill','b');
INSERT INTO people (id, fn, ln) VALUES(4,'Bill','r');
INSERT INTO people (id, fn, ln) VALUES(5,'mike','d');
INSERT INTO people (id, fn, ln) VALUES(6,'mike','c');
INSERT INTO people (id, fn, ln) VALUES(7,'Mike','T');

SELECT fn FROM people GROUP BY LOWER(fn);  /* will not run */
SELECT fn FROM people GROUP BY fn;  /* runs, but not what I want */

Here's what I get:

adam
mike
Mike
bill
Bill

Here's what I want:

Mike
adam
bill

Obviously, there's something I'm missing. And no, I can't just sanitize the data as I put it into the database. What should I read to understand this?

Answer 1

Generally, if you want to select something in the aggregate query, you have to group by this "something". In your case, you can get results you want by selecting lower(fn):

select lower(fn)
from people
group by lower(fn)

Luckily, PostgreSQL allows you to group by alias, so you don't have to repeat lower(fn) twice:

select lower(fn) as lfn
from people
group by lfn

sql fiddle demo

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

select distinct lower(fn)
from people