PostgreSQL，与众不同，按排序和括号

Question

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.

Answer 1

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.