Case-insensitive order while preserving the case of the returned strings I have noticed that when ordering, PostgreSQL (I'm currently working with 11 but I think the same applies to 10 and 9) is case sensitive. It returns upper-cased string before lower-cased one.
For instance, let's say I have the friends table with 3 records and the first_name column contains respectively  adam, Tony, Paul.

SELECT name
FROM friends
ORDER BY name


returns

[name]
 Paul
 Tony
 adam


To get them alphabetically ordered regardless of the case, I can use the lower function.

SELECT name
FROM friends
ORDER BY lower(name)


returns

[name]
 adam
 Paul
 Tony


Now I need to join the friends table with the cities table in order to get only the friends associated with a city while keeping the friends alphabetically ordered and without changing the case of their name .

The following query is valid

SELECT DISTINCT lower(friends.name)
FROM friends
INNER JOIN cities
ON friends.city_id = cities.id
ORDER BY lower(friends.name)


But the returned names are all lowercase.
However, the query below

SELECT DISTINCT friends.name
FROM friends
INNER JOIN cities
ON friends.city_id = cities.id
ORDER BY lower(friends.name)


returns the error


  ORDER BY expressions must appear in select list.


What is the simplest way to apply a case-insensitive order while preserving the case of the returned strings?
The solution is simple: don't use a join. This also removes the need for a DISTINCT.

select f.name
from friends f
where exists (select *
              from cities c
              where f.city_id = c.id)
order by lower(f.name);


Online example: https://rextester.com/SNA62411