Question: I am curious whether Wordpress is able to run nested `meta_query`, with each having different relation keys? As of Wordpress 3.0, `tax_query` is able to perform this function; I'm wondering whether this has an equivalent with `meta_query`.

```
$results = query_posts( array(
    'post_type' => 'event_id',
    'meta_query' => array(
        'relation' => 'AND',
        array(
            'relation' => 'OR',
            array(
                'key' => 'primary_user_id',
                'value' => $user_id
            ),
            array(
                'key' => 'secondary_user_id',
                'value' => $user_id
            )
        ),
        array(
            'key' => 'date',
            'value' => array( $start_date, $end_date ),
            'type' => 'DATETIME',
            'compare' => 'BETWEEN'
        )
    )
) );

```

References:

* [WP\_Query Custom Field Parameters - Multiple Custom Field Handling](http://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters)
* [query multiple taxonomy and show post count](https://wordpress.stackexchange.com/questions/23995/query-multiple-taxonomy-and-show-post-count)
* [How navigation works in custom loop within shortcode?](https://wordpress.stackexchange.com/questions/70010/how-navigation-works-in-custom-loop-within-shortcode)

Answer: 
Meanwhile this is possible, see documentation with example and explanation:

Old link: <https://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters>
Update: 2021, new link: <https://developer.wordpress.org/reference/classes/wp_query/#custom-field-post-meta-parameters>

and another example <https://wordpress.org/support/topic/wp_query-with-multiple-meta_query/#post-9410992>

```
'meta_query'  => array(
    'relation' => 'OR',
    array(
        'relation' => 'AND',
        array(
            'key'     => '_price',
            'value'   => 1,
            'compare' => '>=',
            'type' => 'DECIMAL',
        ),
        array(
            'key'     => '_price',
            'value' => 3000,
            'compare' => '<=',
            'type' => 'DECIMAL',
        ),
    ),
    array(
        'relation' => 'AND',
        array(
            'key'     => '_price',
            'value'   => 3001,
            'compare' => '>=',
            'type' => 'DECIMAL',
        ),
        array(
            'key'     => '_price',
            'value' => 6000, //fixed <= to =>
            'compare' => '<=',
            'type' => 'DECIMAL',
        ),
    )
),

```
That seems to be impossible. Please someone correct me if I'm wrong.

The `meta_query` parameter will actually be transformed into a `WP_Meta_Query` object, and the `relation` verification won't go deeper in [`wp-includes/meta.php`](http://core.trac.wordpress.org/browser/trunk/wp-includes/meta.php#L647), and occurs just once in the top level:

```
if ( isset( $meta_query['relation'] ) && strtoupper( $meta_query['relation'] ) == 'OR' ) {
    $this->relation = 'OR';
} else {
    $this->relation = 'AND';
}

```

A possible solution for this is to build your own JOIN for this query.

```
$query = new WP_Query( array(
    ...
    'my_meta_query' => true,
    'suppress_filters' => false
) );

add_filter( 'posts_join', 'my_meta_query_posts_join', 10, 2 );
function my_meta_query_posts_join( $join, $query ) {

    if ( empty( $query->query_vars['my_meta_query'] ) )
        return $join;

    global $wpdb;

    $new_join = "
        INNER JOIN {$wpdb->postmeta} pm1 ON 1=1
            AND pm1.post_id = {$wpdb->posts}.ID
            AND pm1.meta_key = '_some_meta_key'
            AND pm1.meta_value = 'some_value'
    ";

    return $join . ' ' . $new_join;
}

```

And if you need further verifications and rules, you can also use the `posts_where` filter.