SQL counting distinct matching values

Question

ive been struggling with this for a while:

ive got 2 tables publishers, and students.

im trying to write a query to count how many students share the postccode or "pcode" text value. the data im using has leading leading spaces both before and after, and im meant to take care of that in query as well. another problem is that a few publishers have the same postcode once you trim those spaces from either side, so i need to count the unique trimmed publisher postcodes that are equal to trimmed student postcodes.

im getting some sort of syntax error for a closed bracket on the first line when trying the following and im really stumped

aside from the syntax error im not so certain this will work either

SELECT COUNT(DISTINCT(LTRIM(RTRIM(publishers.pcode)))) As pubpcode 
  FROM publishers, students
 WHERE (LTRIM(RTRIM((publishers.pcode))))) = pubpcode;

Answer 1

 SELECT ltrim(rtrim(pubcode)),count(*) Number_of_Student
 FROM publisher p inner join students s
 on ltrim(rtrim(s.pcode))=ltrim(rtrim(pubcode))
 group by ltrim(rtrim(pubcode));

If you want to count unique students for each pubcode then use distinct

     SELECT ltrim(rtrim(pubcode)),count(distinct s.id) Number_of_Student
     FROM publisher p inner join students s
     on ltrim(rtrim(s.pcode))=ltrim(rtrim(pubcode))
     group by ltrim(rtrim(pubcode));

Answer 2

I would suggest doing union all and then aggregating:

select trim(pcode), sum(is_student), sum(is_pub)
from (select pcode, 1 as is_student, 1 as is_pub
      from students
      union all
      select pcode, 0, 1
      from publisher
     ) sp
group by trim(pcode)