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I know how to get a custom field value for a specific post.

get_post_meta($post_id, $key, $single);

What I need is to get all the values associated with a specific custom post key, across all posts.

Anyone knows of an efficient way to do this? I wouldn't want to loop through all post id's in the DB.

Example:

4 posts all with different values for a custom field called 'Mood'. 2 posts have the value 'happy', 1 post have 'angry' and 1 post has 'sad'

I want to output : across all posts we have: two happy, one angry and one sad author(s).

But for LOTS of posts.

What I'm looking for is either:

  • a WP function to get this. or
  • a custom query to get this as efficiently as possible.
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2  
Seems like you're using this as a taxonomy. Why not simply (automatically) add a term to these posts when saving? Would make querying a lot easier. –  kaiser Feb 15 '11 at 15:11
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5 Answers

up vote 21 down vote accepted

One possible approach would be to use one of the helper methods in the WPDB class to do a more refined meta based query. The caveat to using some of these functions however, is that you don't usually get back a simple array of data and usually have to make needless references to object properties, even if you're only calling for one column or row.

Of course, not all functions are the one and the same, and a purposeful mention goes out to the WPDB method, get_col which returns a simple flat array of the data queried for, i make this mention specifically because the example following will call upon this method.

WordPress - WPDB Selecting a column of data
$wpdb->get_col()

Here's an example function which queries the database for all posts of a chosen post type, post status and with a specific meta key(or custom field to the less technically minded).

function get_meta_values( $key = '', $type = 'post', $status = 'publish' ) {

    global $wpdb;

    if( empty( $key ) )
        return;

    $r = $wpdb->get_col( $wpdb->prepare( "
        SELECT pm.meta_value FROM {$wpdb->postmeta} pm
        LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
        WHERE pm.meta_key = '%s' 
        AND p.post_status = '%s' 
        AND p.post_type = '%s'
    ", $key, $status, $type ) );

    return $r;
}

So for example, if you like to find out which posts have a meta key of rating, for the post type movies and you'd like to store that information inside a variable, an example of such a call would be..

$movie_ratings = get_meta_values( 'rating', 'movies' );

If you wanted to do nothing more than print that data to screen, PHP's implode function can quickly splice that simple array into lines of data.

// Print the meta values seperate by a line break
echo implode( '<br />', get_meta_values( 'YOURKEY' ));

You can also use the returned data to work out how many posts have these meta values by doing a simple loop over the returned data and building an array of the counts, for example.

$movie_ratings = get_meta_values( 'rating', 'movies' );
if( !empty( $movie_ratings ) ) {
    $num_of_ratings = array();
    foreach( $movie_ratings as $meta_value )
        $num_of_ratings[$meta_value] = ( isset( $num_of_ratings[$meta_value] ) ) ? $num_of_ratings[$meta_value] + 1 : 1;
}

/*
Result:
Array(
    [5] => 10
    [9] => 2
)
// ie. there are 10 movie posts with a rating of 5 and 2 movie posts with a rating of 9.
*/

This logic could be applied to various kinds of data, and extended to work any number of different ways. So i hope my examples have been helpful and simple enough to follow.

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It worked for me also. Thank you. –  Jamie Apr 21 '13 at 23:24
    
Still works like a charm. Cheers! –  Ennui Aug 16 '13 at 13:37
    
Happy to hear that. :) –  t31os Jan 24 at 18:13
    
Also fun-fact for future viewers, if you want to pull only Unique meta values - you type DISTINCT right after the SELECT in the function above. Could be useful. –  Howdy_McGee Feb 7 at 18:24
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the fastest way would be a custom sql query and i'm not sure but you can try

$wpdb->get_results("
  SELECT posts.* , COUNT(*) 'moodcount'
  FROM $wpdb->posts as posts
  JOIN $wpdb->postmeta as postmeta
  ON postmeta.post_id = posts.ID
  AND postmeta.meta_key = 'Mood'
  GROUP BY postmeta.meta_key
");

If anything then its a start.

share|improve this answer
    
thanks, but shouldn't custom quesries be avoided 'at all cost'? I'd prefer to use the WP abstraction layer (is that what it's called?)... but of course if this is not possible.. –  mikkelbreum Jun 8 '11 at 10:53
    
Custom queries, if written the right way, can be better and you should only avoid them if you don't know what you're doing. –  Bainternet Jun 8 '11 at 13:07
    
I Agree with mwb .custom queries are very usefull and practical, but I think they are also much heavier on the DB.. especially using SRT functions.. –  krembo99 Dec 10 '11 at 3:20
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I'd just like to add one tiny thing to t31os's code above. I changed "SELECT" into "SELECT DISTINCT" to eliminate duplicate entries when I used this code myself.

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I can imagine cases where it would be valid to have multiple meta values of the same value, and thus didn't not make that addition to my code. If you want distinct values, this would be the way to go though. Additionally you could also add that in as an argument for the function(so you can use it or not, as appropriate). –  t31os Jan 24 at 18:19
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There's no reason why you can't merge t31os and Bainternet's code to have a reusable prepared statement (wordpress style) that returns the count and the values in one efficient operation.

It's a custom query but it's still using the wordpress database abstraction layer - so for example it doesn't matter what the table names really are, or if they change, and it's a prepared statement so we're that much safer from SQL attacks etc.

In this instance I'm no longer checking for post type and I'm excluding empty strings:

    $r = $wpdb->get_results(  $wpdb->prepare( "
        SELECT pm.meta_value AS name, count(*) AS count  FROM {$wpdb->postmeta} pm
        LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
        WHERE pm.meta_key = '%s'
        AND pm.meta_value != '' 
        AND p.post_type = '%s'
        GROUP BY pm.meta_value
        ORDER BY pm.meta_value          
        ", $key, $type) 
        );
    return $r;

In this particular is

This will return an array of objects like so:

array  
 0 => 
 object(stdClass)[359]
  public 'name' => string 'Hamish' (length=6)
  public 'count' => string '3' (length=1)
 1 => 
 object(stdClass)[360]
  public 'name' => string 'Ida' (length=11)
  public 'count' => string '1' (length=1)
 2 => 
 object(stdClass)[361]
  public 'name' => string 'John' (length=12)
  public 'count' => string '1' (length=1)
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For getting all meta values by a meta key

Check wp->db wordpress codex

$values = $wpdb->get_col("SELECT meta_value
    FROM $wpdb->postmeta WHERE meta_key = 'yourmetakey'" );
share|improve this answer
    
The issue with this approach is the lack of specificity, you'll get numerous results from such a query, which could include drafts, trashed items, posts, pages, and any other post type that exists. You should never query for what you don't need, specificity is most certainly required here. –  t31os Jan 24 at 18:23
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