Wordpress Query Parameters Custom Field Parameter

Simple Custom Field Query:
Display posts where the custom field key is ‘color’, regardless of the custom field value:
$query = new WP_Query( array( 'meta_key' => 'color' ) );

Display posts where the custom field value is ‘blue’, regardless of the custom field key:
$query = new WP_Query( array( 'meta_value' => 'blue' ) );

Display page where the custom field value is ‘blue’, regardless of the custom field key:
$args = array(
    'meta_value' => 'blue',
    'post_type'  => 'page'
);
$query = new WP_Query( $args );

Display posts where the custom field key is ‘color’ and the custom field value is ‘blue’:
$args = array(
    'meta_key'   => 'color',
    'meta_value' => 'blue'
);
$query = new WP_Query( $args );

Display posts where the custom field key is ‘color’ and the custom field value IS NOT ‘blue’:
$args = array(
    'meta_key'     => 'color',
    'meta_value'   => 'blue',
    'meta_compare' => '!='
);
$query = new WP_Query( $args );

Display posts where the custom field key is a set date and the custom field value is now. Displays only posts which date has not passed.
$args = array(
    'post_type'    => 'event',
    'meta_key'     => 'event_date',
    'meta_value'   => date( "Ymd" ), // change to how "event date" is stored
    'meta_compare' => '>',
);
$query = new WP_Query( $args );

Display ‘product'(s) where the custom field key is ‘price’ and the custom field value that is LESS THAN OR EQUAL TO 22.
By using the ‘meta_value’ parameter the value 99 will be considered greater than 100 as the data are stored as ‘strings’, not ‘numbers’. For number comparison use ‘meta_value_num’.
$args = array(
    'meta_key'     => 'price',
    'meta_value'   => '22',
    'meta_compare' => '<=',
    'post_type'    => 'product'
);
$query = new WP_Query( $args );

Display posts with a custom field value of zero (0), regardless of the custom field key:
$args = array(
    'meta_value' => '_wp_zero_value'
);
$query = new WP_Query( $args );

Display posts from a single custom field:
$args = array(
    'post_type'  => 'product',
    'meta_query' => array(
        array(
            'key'     => 'color',
            'value'   => 'blue',
            'compare' => 'NOT LIKE',
        ),
    ),
);
$query = new WP_Query( $args );
(Note that meta_query expects nested arrays, even if you only have one query.)

Display posts from several custom fields:
$args = array(
    'post_type'  => 'product',
    'meta_query' => array(
        array(
            'key'     => 'color',
            'value'   => 'blue',
            'compare' => 'NOT LIKE',
        ),
        array(
            'key' => 'price',
            'value'   => array( 20, 100 ),
            'type'    => 'numeric',
            'compare' => 'BETWEEN',
        ),
    ),
);
$query = new WP_Query( $args );

Display posts that have meta key ‘color’ NOT LIKE value ‘blue’ OR meta key ‘price’ with values BETWEEN 20 and 100:
$args = array(
    'post_type'  => 'product',
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key'     => 'color',
            'value'   => 'blue',
            'compare' => 'NOT LIKE',
        ),
        array(
            'key'     => 'price',
            'value'   => array( 20, 100 ),
            'type'    => 'numeric',
            'compare' => 'BETWEEN',
        ),
    ),
);
$query = new WP_Query( $args );

The 'meta_query' clauses can be nested in order to construct complex queries. For example, show productss where color=orange OR color=red&size=small translates to the following:
$args = array(
    'post_type'  => 'product',
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key'     => 'color',
            'value'   => 'orange',
            'compare' => '=',
        ),
                array(
                        'relation' => 'AND',
                        array(
                                'key' => 'color',
                                'value' => 'red',
                                'compare' => '=',
                        ),
                        array(
                                'key' => 'size',
                                'value' => 'small',
                                'compare' => '=',
                        ),
        ),
    ),
);
$query = new WP_Query( $args );