Yii: Select 20 last entries order by id ASC

问题

I would like to get the 20 last entries of my table but order by ascending id.

In Sql it's not very complicated:

SELECT * 
FROM (SELECT * FROM comments
      WHERE postID='$id' 
      ORDER BY id DESC 
      LIMIT 20) t
ORDER BY id ASC;

But I would like to to it with my yii model like:

Comment::model()->findAll($criteria)

But i really don't know what I should put in my CDbCriteria!

回答1:

There is a way without using array_reverse, if you think of using this sql:

SELECT * FROM `comments` `t` 
WHERE id 
in (SELECT id 
     FROM (SELECT id FROM comments Where postID = xyz ORDER BY id DESC LIMIT 20) 
    as q) 
ORDER BY id ASC

which in criteria will become:

$criteria=new CDbCriteria();
$criteria->condition='id in (SELECT id FROM (SELECT id FROM comments Where postID='.$id.' ORDER BY id DESC LIMIT 20) as q)';
$criteria->order='id ASC';

Update:

With your original query, you could have also used findBySql :

$sql='SELECT * FROM (SELECT * FROM comments  WHERE postID= :postid  ORDER BY id DESC LIMIT 20) q ORDER BY id ASC';
$params=array('postid'=>$id);
$comments=Comment::model()->findAllBySql($sql,$params);

The performance of this query was better than my previous query.

回答2:

$models = Comment::model()->findAll(array(
    "condition" => "WHERE postID = '".$id."'",
    "order" => "id DESC",
    "limit" => 20,
));

~~Will get the last 20. And now you want to order that record set by id ASC correct? Is there not another field you can order by for a similar result (maybe a date or created field?) eg:~~

"order" => "id DESC, created ASC"

Scrap that secondary ordering, but why not just use array reverse?

$models = array_reverse($models);

回答3:

UPD:

Please note, that in general, some other solutions are better than mine.

Using offset can decrease performance of your queries. See: http://www.slideshare.net/Eweaver/efficient-pagination-using-mysql and Why does MYSQL higher LIMIT offset slow the query down?

So, when the number of Comments will increase, you can get performance degradation.

What about using offset feature?

    $model = Comment::model();

    $condition = 'postID =' . $id;
    $limit = 20;
    $totalItems = $model->count($condition);

    $criteria = new CDbCriteria(array(
        'condition' => $condition,
        'order' => 'id ASC',
        'limit' => $limit,
        'offset' => $totalItems - $limit // if offset less, thah 0 - it starts from the beginning
    ));

    $result = $model->findAll($criteria);

来源：https://stackoverflow.com/questions/12941979/yii-select-20-last-entries-order-by-id-asc

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Yii