Question

I’m a bit stuck trying to figure out how to design an Oracle query, and although there are similar questions here none of them seem to quite address the issues I’m facing.

I have two tables and I want to join them:

PROJECT table
  PROJECT_ID   TITLE
  101          First project
  102          Second project
  103          Third project
  104          Fourth project
  105          Fifth project

EVENT table
  EVENT_ID  PROJECT_FK  EVENT_TYPE  EVENT_DATE  EVENT_DESC
  201       101         301         2010-01-01  First event
  202       101         301         2010-01-01  Second event
  203       101         302         2010-01-02  Third event
  204       102         301         2010-01-03  Fourth event
  205       102         301         2010-01-04  Fifth event
  206       104         301         2010-01-05  Sixth event
  207       105         302         2010-01-06  Seventh event

I would like to get a list of each project’s data (from the PROJECT table) along with details of the most recent event, but only events of a single type (all other events should be ignored.) One row, and only one row, should be returned for each project (so if multiple matching events have the same date either one is fine, and if there are no events nulls/blanks should be returned in the event fields.)

This is what the output might look like:

SELECT <???> WHERE PROJECT_ID IN (101, 102, 103, 105)   /* for event type 301 only */

  PROJECT_ID   TITLE           EVENT_DATE  EVENT_DESC
  101          First project   2010-01-01  First event
  102          Second project  2010-01-04  Fifth event
  103          Third project   NULL        NULL
  105          Fifth project   NULL        NULL

I’m finding this quite tricky as the only examples I can find either assume that the max(date) is unique (but here selecting by that will return the wrong row) or they assume there is a lot of duplication so GROUP BY will work.

Answer 1

Oracle 9i+, using ROW_NUMBER:

SELECT x.project_id,
       x.title,
       x.event_date,
       x.event_desc
  FROM (SELECT p.project_id,
               p.title,
               e.event_date,
               e.event_desc,
               ROW_NUMBER() OVER(PARTITION BY p.project_id
                                     ORDER BY e.event_date) AS rank
          FROM PROJECT p
     LEFT JOIN EVENT e ON e.project_fk = p.project_id
                      AND e.event_type = 301
         WHERE p.project_id IN (101,102,103)) x
 WHERE x.rank = 1

Oracle 9i+, Using WITH and ROW_NUMBER:

WITH example AS (
     SELECT p.project_id,
            p.title,
            e.event_date,
            e.event_desc,
            ROW_NUMBER() OVER(PARTITION BY p.project_id
                                  ORDER BY e.event_date) AS rank
       FROM PROJECT p
  LEFT JOIN EVENT e ON e.project_fk = p.project_id
                   AND e.event_type = 301
      WHERE p.project_id IN (101,102,103))
SELECT x.project_id,
       x.title,
       x.event_date,
       x.event_desc
  FROM example x
 WHERE x.rank = 1