Query returning exact number of rows

Question

I have a table that stores two foreign keys, implementing a n:m relationship.

One of them points to a person (subject), the other one to a specific item.

Answer 1

Could work like this (pure SQL solution):

SELECT a.sal_subject
     , b.sub_item
FROM  (
    SELECT generate_series(1, max_items) AS rn
         , sal_subject
    FROM   subject
    ) a
LEFT   JOIN (
    SELECT row_number() OVER (PARTITION BY sal_subject ORDER BY sub_item) AS rn
         , sal_subject
         , sub_item
    FROM   subject_items
    ) b USING (sal_subject, rn)
ORDER  BY sal_subject, rn

1. Generate the maximum rows per subject, let's call them theoretical items.
   See the manual for generate_series().
2. Apply a row-number to existing items per subject.
   Manual about window functions.
3. LEFT JOIN the existing items to the theoretical items per subject. Missing items are filled in with NULL.

In addition to the table you disclosed in the question, I assume a column that holds the maximum number of items in the subject table:

CREATE temp TABLE subject
( sal_subject integer,     -- primary key of subject
  max_items int);          -- max. number of items

---

Query for PostgreSQL 8.3, substituting for the missing window function row_number():

SELECT a.sal_subject
     , b.sub_item
FROM  (
    SELECT generate_series(1, max_items) AS rn
         , sal_subject
    FROM   subject
    ) a
LEFT   JOIN (
    SELECT rn, sal_subject, arr[rn] AS sub_item
    FROM  (
        SELECT generate_series(1, ct) rn, sal_subject, arr
        FROM  (
            SELECT s.sal_subject
                 , s.ct
                 , ARRAY(
                        SELECT sub_item
                        FROM   subject_items s0
                        WHERE  s0.sal_subject = s.sal_subject
                        ORDER  BY sub_item
                    ) AS arr
            FROM  (
                SELECT sal_subject
                     , count(*) AS ct
                FROM   subject_items
                GROUP  BY 1
                ) s
            ) x
        ) y
    ) b USING (sal_subject, rn)
ORDER  BY sal_subject, rn

More about substituting row_number() in this article by Quassnoi.