Question

We’re currently migrating our database back-end from Firebird to PostgreSQL. We use NHibernate as an ORM and the transition has been quite painless albeit a few differences in SQL.

We have one query which we execute using NHibernate.CreateSQLQuery as we build the SQL ourselves. The query returns a string List of the most 5 most recently accessed Ninja names for the passed in UserID.

This was no problem in Firebird using the following SQL;

 SELECT FIRST 5 DISTINCT NI.NINJA_NAME
 FROM NINJA_ACCESS NA 
      INNER JOIN NINJAS NI ON NA.NINJA_ID = NI.NINJA_ID
 WHERE NA.USER_ID = 1
 ORDER BY NI.NINJA_ACCESS_DATE DESC

Returning an ordered (by access date descending) list of strings.

Now in PostgreSQL we are encountering problems with this simple query;

 SELECT DISTINCT ON (NI.NINJA_NAME) NI.NINJA_NAME
 FROM NINJA_ACCESS NA 
      INNER JOIN NINJAS NI ON NA.NINJA_ID = NI.NINJA_ID
 WHERE NA.USER_ID = 1
 ORDER BY NI.NINJA_ACCESS_DATE DESC
 LIMIT 5

It appears that we cannot return a single “distinct” column of data with ordering being performed on a separate column that isn’t included in the select. Any variation in the above SQL doesn’t work unless we include the Access_Date column in the select.

Any ideas how we can resolve this simple query in PostgreSQL?

Side-note: This is an important query for our Ninja Management System, the Ninja’s insist that it work!!!

Answer 1

This query should be the equivalent:

SELECT n.ninja_name
      ,max(n.ninja_access_date) AS max_access_date
FROM   ninja_access na 
JOIN   ninjas n USING (ninja_id)
WHERE  na.user_id = 1
GROUP  BY n.ninja_name
ORDER  BY 2 DESC
LIMIT  5;

• ORDER BY 2 is just shorthand for ORDER BY max(n.ninja_access_date).
• We need the max(n.ninja_access_date) to get the most recent entry per ninja_name.
  And sort descending to get the most recent names.
• JOIN ninjas n USING (ninja_id) is shorthand for
  JOIN ninjas n ON n.ninja_id = na.ninja_id

You don’t have to include the access date in the SELECT list. To only get the names:

SELECT n.ninja_name
FROM   ninja_access na 
JOIN   ninjas ni USING (ninja_id)
WHERE  na.user_id = 1
GROUP  BY n.ninja_name
ORDER  BY max(n.ninja_access_date) DESC
LIMIT  5;