Unique rows from PostgreSQL database and ordering - sql

Question

how can I get unique rows from PostgreSQL (8.4.20), at this moment my query looks like:
SELECT
DISTINCT ON(student.id) student.id,
student.*,
programme_stage.*
FROM
person AS student
INNER JOIN
programme ON (student.id = programme.person_id)
RIGHT JOIN
programme_stage ON (programme.id = programme_stage.programme_id)
ORDER BY
student.id,
student.last_name ASC,
student.first_name ASC
LIMIT 10 OFFSET 0
The above query works correctly, but I want to sort by the last and first name in first order.
Sample data:
| id | first_name | last_name | programme_id | programme_stage _id |
|----|------------|-----------|--------------|---------------------|
| 1 | Michał | Nowak | 1 | 1 |
| 2 | Jan | Kowalski | 2 | 2 |
| 3 | Tomasz | Thomas | 2 | 1 |
Expected output:
| id | first_name | last_name | programme_id | programme_stage _id |
|----|------------|-----------|--------------|---------------------|
| 2 | Jan | Kowalski | 2 | 2 |
| 1 | Michał | Nowak | 1 | 1 |
| 3 | Tomasz | Thomas | 2 | 1 |
If I try to remove from order statment student.id column, I getting error:
..SELECT DISTINCT ON expressions must match the expressions .. ORDER BY LINE 2: DISTINCT ON(student_person.id) student_person.id,
^

Answer 1

Use a subquery:
SELECT s.*
FROM (SELECT DISTINCT ON (s.id) s.*, ps.*
FROM programme_stage ps LEFT JOIN
programme p
ON programme.id = ps.programme_id LEFT JOIN
person s
ON s.id = p.person_id)
ORDER BY s.id
LIMIT 10 OFFSET 0
) s
ORDER BY s.last_name ASC, s.first_name ASC;
Notes:
I prefer shorter table aliases so the query is easier to write and to read.
I prefer LEFT JOIN to RIGHT JOIN. However, because you are using the person table for aggregation, you probably want inner joins.
There is no need to select the id twice.
You may need to select explicit columns instead of * if columns have the same name.