Question

I have a parent table with entries for documents and I have a history table which logs an audit entry every time a user accesses one of the documents.

I’m writing a search query to return a list of documents (filtered by various criteria) with the latest user id to access each document returned in the result set.

Thus for

     DOCUMENTS     ID | NAME     1  | Document 1     2  | Document 2     3  | Document 3     4  | Document 4     5  | Document 5      HISTORY     DOC_ID | USER_ID | TIMESTAMP     1      | 12345   | TODAY     1      | 11111   | IN THE PAST     1      | 11111   | IN THE PAST     1      | 12345   | IN THE PAST     2      | 11111   | TODAY     2      | 12345   | IN THE PAST     3      | 12345   | IN THE PAST 

I’d be looking to get a return from my search like

     ID | NAME       | LAST_USER_ID     1  | Document 1 | 12345     2  | Document 2 | 11111     3  | Document 3 | 12345     4  | Document 4 |      5  | Document 5 |  

Can I easily do this with one SQL query and a join between the two tables?

Answer 1

Revising what Andy White produced, and replacing square brackets (MS SQL Server notation) with DB2 (and ISO standard SQL) ‘delimited identifiers’:

SELECT d.id, d.name, h.last_user_id     FROM Documents d LEFT JOIN          (SELECT r.doc_id AS id, user_id AS last_user_id               FROM History r JOIN                    (SELECT doc_id, MAX('timestamp') AS 'timestamp'                         FROM History                         GROUP BY doc_id                    ) AS l                    ON  r.'timestamp' = l.'timestamp'                    AND r.doc_id      = l.doc_id          ) AS h          ON d.id = h.id 

I’m not absolutely sure whether ‘timestamp’ or ‘TIMESTAMP’ is correct – probably the latter.

The advantage of this is that it replaces the inner correlated sub-query in Andy’s version with a simpler non-correlated sub-query, which has the potential to be (radically?) more efficient.