WordPress 4.2 is coming soon, and Boone Gorges writes on Make WordPress Core about WP_Query improvements for ‘orderby’ and ‘meta_query’:

Query improvements in WP 4.2: ‘orderby’ and ‘meta_query’

WP 4.2 will include a number of improvements in the way query classes handle ‘orderby’, particularly when used in connection with the ‘meta_query’ parameter.

WP_Query has long supported the sorting of results by postmeta, using the following syntaxes:

$q1 = new WP_Query( array(
    'meta_key' => 'foo',
    'orderby' => 'meta_value',
) );
$q2 = new WP_Query( array(
    'meta_key' => 'foo',
    'orderby' => 'foo',
) );

These arguments have always operated more or less independently from ‘meta_query’. Internally, ‘meta_key’ and ‘meta_value’ are converted to the first clause in a ‘meta_query’, but this conversion is opaque as far as ‘orderby’ is concerned.

WP 4.2 will address this shortcoming with a new syntax for ordering by specific clauses of a ‘meta_query’ array. When building the meta_query’ parameter, use explicit array indexes for the meta query clauses. These indexes will be used as handles, which can be referenced from the ‘orderby’ param. For example:

$q = new WP_Query( array(
    'meta_query' => array(
        'relation' => 'AND',
        'state_clause' => array(
            'key' => 'state',
            'value' => 'Wisconsin',
        'city_clause' => array(
            'key' => 'city',
            'compare' => 'EXISTS',
    'orderby' => 'city_clause', // Results will be ordered by 'city' meta values.
) );

This new syntax can be used in conjunction with complex ‘orderby’ parameters:

// ...
    'orderby' => array(
        'city_clause' => 'ASC',
        'state_clause' => 'DESC',
// ...

Read more at Make WordPress Core.

This may interest you:   Display commas in WordPress tags