PostgreSQL，从2个表中选择，但仅从表2中选择最新元素

Question

Hey, I have 2 tables in PostgreSql:

1 - documents: id, title
2 - updates: id, document_id, date

and some data:

documents:

| 1 | Test Title |

updates:

| 1 | 1 | 2006-01-01 |
| 2 | 1 | 2007-01-01 |
| 3 | 1 | 2008-01-01 |

So All updates are pointing to the same document, but all with different dates for the updates.

What I am trying to do is to do a select from the documents table, but also include the latest update based on the date.

How should a query like this look like? This is the one I currently have, but I am listing all updates, and not the latest one as the one I need:

SELECT * FROM documents,updates WHERE documents.id=1 AND documents.id=updates.document_id ORDER BY date

To include; The reason I need this in the query is that I want to order by the date from the updates template!

Edit: This script is heavily simplified, so I should be able to create a query that returns any number of results, but including the latest updated date. I was thinking of using a inner join or left join or something like that!?

Answer 1

Use PostgreSQL extension DISTINCT ON :

SELECT  DISTINCT ON (documents.id) *
FROM    document
JOIN    updates
ON      updates.document_id = document_id
ORDER BY
        documents.id, updates.date DESC

This will take the first row from each document.id cluster in ORDER BY order.

Test script to check:

SELECT  DISTINCT ON (documents.id) *
FROM    (
        VALUES
        (1, 'Test Title'),
        (2, 'Test Title 2')
        ) documents (id, title)
JOIN    (
        VALUES
        (1, 1, '2006-01-01'::DATE),
        (2, 1, '2007-01-01'::DATE),
        (3, 1, '2008-01-01'::DATE),
        (4, 2, '2009-01-01'::DATE),
        (5, 2, '2010-01-01'::DATE)
        ) updates (id, document_id, date)
ON      updates.document_id = documents.id
ORDER BY
        documents.id, updates.date DESC

Answer 2

You may create a derived table which contains only the most recent "updates" records per document_id, and then join "documents" against that:

SELECT d.id, d.title, u.update_id, u."date"
FROM documents d
LEFT JOIN
-- JOIN "documents" against the most recent update per document_id
(
SELECT recent.document_id, id AS update_id, recent."date"
FROM updates
INNER JOIN
(SELECT document_id, MAX("date") AS "date" FROM updates GROUP BY 1) recent
ON updates.document_id = recent.document_id
WHERE
  updates."date" = recent."date"
) u
ON d.id = u.document_id;

This will handle "un-updated" documents, like so:

pg=> select * from documents;
 id | title 
----+-------
  1 | foo
  2 | bar
  3 | baz
(3 rows)

pg=> select * from updates;
 id | document_id |    date    
----+-------------+------------
  1 |           1 | 2009-10-30
  2 |           1 | 2009-11-04
  3 |           1 | 2009-11-07
  4 |           2 | 2009-11-09
(4 rows)

pg=> SELECT d.id ...
 id | title | update_id |    date    
----+-------+-----------+------------
  1 | foo   |         3 | 2009-11-07
  2 | bar   |         4 | 2009-11-09
  3 | baz   |           | 
(3 rows)

Answer 3

select *
from documents
left join updates
  on updates.document_id=documents.id
  and updates.date=(select max(date) from updates where document_id=documents.id)
where documents.id=?;

It has the some advantages over previous answers:

• you can write document_id only in one place which is convenient;
• you can omit where and you'll get a table of all documents and their latest updates;
• you can use more broad selection criteria, for example where documents.id in (1,2,3) .

You can also avoid a subselect using group by, but you'll have to list all fields of documents in group by clause:

select documents.*, max(date) as max_date
  from documents
  left join updates on documents.id=document_id
  where documents.id=1
  group by documents.id, title;

Answer 4

From the top of my head:

ORDER BY date DESC LIMIT 1

If you really want only id 1 your can use this query:

SELECT * FROM documents,updates 
    WHERE documents.id=1 AND updates.document_id=1 
    ORDER BY date DESC LIMIT 1

http://www.postgresql.org/docs/8.4/interactive/queries-limit.html

Answer 5

This should also work

SELECT * FROM documents, updates 
    WHERE documents.id=1 AND updates.document_id=1
    AND updates.date = (SELECT MAX (date) From updates)