As @StephenHarris pointed out in the comments, there's a core function that might come in handy when building the SQL Query: get_meta_sql() - Examples in Codex. Basically this function is just used to build the meta SQL statement that gets used in WP_Query, but you can use it in this case (or others) as well. The argument that you throw into it is an array, the exact same that would add to a WP_Query.

As @StephenHarris pointed out in the comments, there's a core function that might come in handy when building the SQL Query: get_meta_sql() - Examples in Codex. Basically this function is just used to build the meta SQL statement that gets used in WP_Query, but you can use it in this case (or others) as well. The argument that you throw into it is an array, the exact same that would add to a WP_Query.

Dependencies to keep in mind

Automatic query building

As @StephenHarris pointed out in the comments, there's a core function that might come in handy when building the SQL Query: get_meta_sql() - Examples in Codex. Basically this function is just used to build the meta SQL statement that gets used in WP_Query, but you can use it in this case (or others) as well. The argument that you throw into it is an array, the exact same that would add to a WP_Query.

$meta_sql = get_meta_sql(
    $meta_query,
    'post',
    $wpdb->posts,
    'ID'
);

The return value is an array:

$sql => (array) 'join' => array(),
        (array) 'where' => array()

So you can use $sql['join'] and $sql['where'] in your callback.

Dependencies to keep in mind

• "get_{$adjacent}_post_join"

    $join // Only if `$in_same_cat || ! empty( $excluded_categories` and then: " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id"; AND IF $in_same_cat then it APPENDS: " AND tt.taxonomy = 'category' AND tt.term_id IN (" . implode(',', $cat_array) . ")";
    $in_same_cat
    $excluded_categories
  

• "get_{$adjacent}_post_where"

    $wpdb->prepare(
         "
          WHERE p.post_date $op %s // $op = $previous ? '<' : '>'; | $current_post_date
          AND p.post_type = %s     // $post->post_type
          AND p.post_status = 'publish' $posts_in_ex_cats_sql // $posts_in_ex_cats_sql = " AND tt.taxonomy = 'category' AND tt.term_id NOT IN (" . implode($excluded_categories, ',') . ')'; OR empty string if $in_same_cat || ! empty( $excluded_categories
         "
        ,$current_post_date
        ,$post->post_type
    )
    $in_same_cat
    $excluded_categories
  

• "get_{$adjacent}_post_sort"

    "ORDER BY p.post_date $order LIMIT 1"`
  

• "get_{$adjacent}_post_join"

    $join
    // Only if `$in_same_cat`
    // or: ! empty( $excluded_categories` 
    // and then: 
    // " INNER JOIN $wpdb->term_relationships AS tr 
    //     ON p.ID = tr.object_id 
    // INNER JOIN $wpdb->term_taxonomy tt 
    //     ON tr.term_taxonomy_id = tt.term_taxonomy_id";  
    // and if $in_same_cat then it APPENDS: 
    // " AND tt.taxonomy = 'category' 
    // AND tt.term_id IN (" . implode(',', $cat_array) . ")";
    $in_same_cat
    $excluded_categories
  

• "get_{$adjacent}_post_where"

    $wpdb->prepare(
          // $op = $previous ? '<' : '>'; | $current_post_date
           "WHERE p.post_date $op %s "
          // $post->post_type
          ."AND p.post_type = %s "
          // $posts_in_ex_cats_sql = " AND tt.taxonomy = 'category' 
          // AND tt.term_id NOT IN (" . implode($excluded_categories, ',') . ')'; 
          // OR empty string if $in_same_cat || ! empty( $excluded_categories
          ."AND p.post_status = 'publish' $posts_in_ex_cats_sql "
        ",
        $current_post_date,
        $post->post_type
    )
    $in_same_cat
    $excluded_categories
  

• "get_{$adjacent}_post_sort"

    "ORDER BY p.post_date $order LIMIT 1"`