1

I list comments for an entry and I allow for user's to edit existing comments and it keeps track of those revisions.

Table structure:

comments: id, summary

revisions: comment_id, revision_id, timestamp

revisions.comment_id = comments.id

revisions.revision_id = the id of the new comment, further explanation: when they select the existing comment, it will display an edit form and they may enter a new comment and submit it. It will insert it into the comments table as a new comment, grab the last id from that table and set it as revision_id in the revisions table.

I want to do a select distinct on the revisions table (to retrieve multiple comment_id, revision_id into an array to be used within the application):

for example: 

select distinct comment_id from revisions

But is it possible to select the appropriate record based on the most recent comment (revisions.timestamp)?

in theory:

select distinct comment_id from revisions WHERE timestamp IS THE LARGEST

revisions table example:

comment_id   revision_id   timestamp
         2            12   20120222180000
         2            13   20120222170000
         5            18   20120222190000
         5            19   20120222200000

In this example of 4 rows, I'd want the query to return two rows,

that being:

comment_id = 2, revision_id = 12 timestamp = 20120222180000

and

comment_id = 5, revision_id = 19 timestamp = 20120222200000

UPDATE: this seems to do the trick, but please let me know if there is a better way

SELECT 
 distinct comment_id, max(timestamp)
FROM
 revisions 
GROUP BY
 comment_id

UPDATE: I also need to include revision_id, the above query only includes comment_id and timestamp

This did it:

SELECT 
 distinct a.comment_id as comment_id, 
 a.revision_id revision_id,
 a.timestamp as timestamp
FROM
 REVISIONS a 
WHERE
 a.timestamp = ( 
 SELECT
  max(b.timestamp)
 FROM
  revisions b
 WHERE
  b.comment_id = a.comment_id
)
4

1 回答 1

0

如果我理解正确，您只需按时间戳降序排序并获取第一个结果。

SELECT comment_id FROM revisions ORDER BY timestamp DESC LIMIT 0,1

编辑：我想我现在明白了，你真正想要什么，试试这个：

SELECT * FROM revisions GROUP BY comment_id ORDER BY timestamp DESC
于 2012-02-22T19:36:41.937 回答