I have the following request:
select distinct m1.firstname, m1.surname
from cd.members as m1
join cd.members as m2 on m2.recommendedby = m1.memid
order by m1.surname, m1.firstname;
and it works fine. But this one:
select distinct m1.firstname, m1.surname
from cd.members as m1
join cd.members as m2 on m2.recommendedby = m1.memid
order by (m1.surname, m1.firstname);
gives me the error:
ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
I can't understand when I have to use parentheses and when not.
Postgres has the concept of tuples or composite types . These are scalar values that appear together -- much like a record or struct in many programming languages.
You can use tuples for expressions, such as:
where (m1.surname, m1.firstname) in ( ('a', 'b'), ('x', 'y') )
The problem with your order by
expression is that the tuple is not in the select
. You could solve this by using parentheses there too:
select distinct (m1.firstname, m1.surname)
from cd.members m1 join
cd.members m2
on m2.recommendedby = m1.memid
order by (m1.surname, m1.firstname);
But I would stick with the parentheses-less version, which is standard SQL and works in all databases.
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