Check if meta key value already exists

Let’s say that I have few posts with meta key “videoid”. Value of that meta key is different for every post (and has to be different). When adding new post, how can I check if videoid with that exact value already exist for some older posts? I used this as a refference for adding custom meta box to admin area of WP: http://codex.wordpress.org/Function_Reference/add_meta_box

Solutions Collecting From Web of "Check if meta key value already exists"

Just do a query with WP_Query using the Custom Fields Parameter – meta_query to look for posts with the meta key and the value – exemplary code:

 // args to query for your key
 $args = array(
   'post_type' => 'your_post_type',
   'meta_query' => array(
       array(
           'key' => 'videoid',
           'value' => '111'
       )
   ),
   'fields' => 'ids'
 );
 // perform the query
 $vid_query = new WP_Query( $args );

 $vid_ids = $vid_query->posts;

 // do something if the meta-key-value-pair exists in another post
 if ( ! empty( $vid_ids ) ) {
     // do your stuff
 }

There is no need to use query_post() – see:
When should you use WP_Query vs query_posts() vs get_posts()?
. If you need a complete array of post objects, not just the ids, remove 'fields' => 'ids'.

ialocin’s answer incorrectly states that the wp_query stored as a variable would just spit out an array of ID’s. Instead it gives the whole WP_Query object, so you have to use ->posts to get that array of post ID’s.

// args to query for your key
 $args = array(
   'post_type' => 'YOUR_POST_TYPE',
   'meta_query' => array(
       array(
           'key' => 'YOUR_META_FIELD_NAME',
           'value' => '111'
       )
   ),
   'fields' => 'ids'
 );
 // perform the query
 $query = new WP_Query( $args );
 $duplicates = $query->posts;

 // do something if the key-value-pair exists in another post
 if ( ! empty( $duplicates ) ) {
     // do your stuff
 }

Found it:

$args = array(
    'meta_query' => array(
        array(
            'key' => 'videoid',
            'value' => $_POST['videoid']
        )
    )
);

$videoQuery = new WP_Query( $args ); 

if ( $videoQuery->have_posts() ) :
    while ( $videoQuery->have_posts() ) : 
            $videoQuery->the_post(); ?>
    echo "<h3 class='post-title'>" . the_title() . "</h3>";
    endwhile; 
    endif;

Or wrap it up in a function:

function meta_value_exists($your_meta_value) {
    $args = array(
        'post_type'   => 'YOUR_POST_TYPE',
        'post_status' => 'publish',
        'numberposts' => 1,
        'meta_key'     => 'your_meta_field',
        'meta_value'   => $your_meta_value,
    );
    $current_post = get_posts($args);
    if( $current_post ) {
        return true;
    } else {
        return false;
    }
}

So then you could just check on your meta value:

$video_id = 1234;
if(meta_value_exists($video_id){
   // do something if exists
} else {
   // do something if not exists
}