WordPress meta queries

If you want to get a post with meta key show_on_homepage and meta value on, you can do it in the following way:

$rd_args = array(
	'meta_key' => 'show_on_homepage',
	'meta_value' => 'on'
);

$rd_query = new WP_Query($rd_args);

If you need to query all posts except the ones with this pair of meta key and value, you can use the following parameters:

$rd_args = array(
	'meta_key' => 'show_on_homepage',
	'meta_value' => 'on',
	'meta_compare' => '!='
);

$rd_query = new WP_Query( $rd_args );

Get Posts with a Specific Custom Field Value

// the meta_key 'color' with the meta_value 'white'
$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'color',
			'value' => 'white'
		)
	)
); 
$rd_query = new WP_Query( $rd_args );

Get all the posts except the ones with meta key «color» and meta value «white»:

$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'color',
			'value' => 'white',
			'compare' => '!='
		)
	)
);

$rd_query = new WP_Query( $rd_args );

Get all the posts with white OR green color custom field value:

// custom field name is color and custom field value is 'white' OR 'green'
$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'color',
			'value' => array('white','green'),
			'compare' => 'IN'
		)
	)
); 
$rd_query = new WP_Query( $rd_args );

Get all the posts (products in online shop for example) except white products and green products:

$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'color',
			'value' => array('white','green'),
			'compare' => 'NOT IN'
		)
	)
);
$rd_query = new WP_Query( $rd_args );

Get Posts Within a Given Range of Numeric Meta Values

// the product price is more than 2000 and less than 4000
$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'price',
			'value' => array( 2000, 4000 ),
			'type' => 'numeric',
			'compare' => 'BETWEEN'
		)
	)
);
$rd_query = new WP_Query( $rd_args );

Numeric Comparison

$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'price',
			'value' => 2000,
			'type' => 'numeric',
			'compare' => '>='
		)
	)
);

The product price is less than 4000:

$rd_args = array(
	'meta_query' => array(
		array(
			'key' => 'price',
			'value' => 4000,
			'type' => 'numeric',
			'compare' => '<'
		)
	)
); 
$rd_query = new WP_Query( $rd_args );

Query Posts by Several (two or more) Custom Field Values

$rd_args = array(
	'meta_query' => array(
		'relation' => 'AND',
		array(
			'key' => 'color',
			'value' => 'white'
		),
		array(
			'key' => 'price',
			'value' => array( 2000, 4000 ),
			'type' => 'numeric',
			'compare' => 'BETWEEN'
		)
	)
); 
$rd_query = new WP_Query( $rd_args );