$meta_query = array( 'relation' => 'OR', array( 'relation' => 'OR', array( 'relation' => 'OR', array( 'relation' => 'OR', array( 'relation' => 'AND', array( 'key' => 'age_min', 'compare' => 'NOT EXISTS', ), array( 'key' => 'age_max', 'compare' => 'NOT EXISTS', ), ), array( 'key' => 'age_from', 'value' => 'foobar', 'compare' => '=', ), array( 'key' => 'age_to', 'value' => 'foobar', 'compare' => '=', ), ), array( 'relation' => 'AND', array( 'key' => 'age_from', 'value' => '$ageMin', 'compare' => '<=', ), array( 'key' => 'age_to', 'value' => '$ageMax', 'compare' => '>=', ), ), ), array( 'relation' => 'AND', array( 'key' => 'age_from', 'value' => '$ageMin', 'compare' => '<=', ), array( 'key' => 'age_to', 'value' => '$ageMax', 'compare' => '>=', ), ), ), array( 'relation' => 'AND', array( 'key' => 'age_from', 'value' => '$ageMin', 'compare' => '<=', ), array( 'key' => 'age_to', 'value' => '$ageMax', 'compare' => '>=', ), ), );