gistfile1.php

$res = $conn->query('
    SET @num := 0, @genre := "";

    SELECT
        id_artista,
        artista,
        art_url,
        genero_url,
        genero,
        (
            SELECT id_foto
            FROM letras.fotos AS f
            WHERE f.id_artista = id_artista
            ORDER BY RAND() LIMIT 1
        ) AS id_foto
    FROM (
        SELECT *,
            @num := if(@genre = genero_url, @num + 1, 1) as total,
            @genre := genero_url as dummy
        FROM(
          SELECT
            a.id_artista,
            a.artista,
            a.dns AS art_url,
            g.dns AS genero_url,
            g.genero
          FROM letras.artistas_generos AS ag
          INNER JOIN letras.artistas AS a ON a.id_artista = ag.id_artista
          INNER JOIN letras.generos AS g ON g.id_genero = ag.id_genero
          INNER JOIN views_artistas_pt AS vap ON a.id_artista = vap.id_artista
          WHERE g.topo = 1 and ag.principal = 1
          ORDER BY g.genero ASC, vap.semanahits DESC
        ) AS y
    ) AS x WHERE x.total <= 5;
');

$genres = $res->fetchAll(\PDO::FETCH_ASSOC);
$res->closeCursor();

$res = $conn->query('SET @num := 0, @genre := "";');
$res = $conn->query('
    SELECT
        id_artista,
        artista,
        art_url,
        genero_url,
        genero
    FROM (
        SELECT *,
            @num := if(@genre = genero_url, @num + 1, 1) as total,
            @genre := genero_url as dummy
        FROM(
          SELECT
            a.id_artista,
            a.artista,
            a.dns AS art_url,
            g.dns AS genero_url,
            g.genero
          FROM letras.artistas_generos AS ag
          INNER JOIN letras.artistas AS a ON a.id_artista = ag.id_artista
          INNER JOIN letras.generos AS g ON g.id_genero = ag.id_genero
          INNER JOIN views_artistas_pt AS vap ON a.id_artista = vap.id_artista
          WHERE g.topo = 1 and ag.principal = 1
          ORDER BY g.genero ASC, vap.semanahits DESC
        ) AS y
    ) AS x WHERE x.total <= 5;
');

$artistas = $res->fetchAll(\PDO::FETCH_ASSOC);
$list = array();
// Apenas os IDs dos artistas
$list = array_map(function($artista) {
    return $artista['id_artista'];
}, $artistas);

$res = $conn->query('
    SELECT
        id_artista,
        (
            SELECT id_foto
            FROM letras.fotos AS f
            WHERE f.id_artista = id_artista
            ORDER BY RAND() LIMIT 1
        ) AS foto
    FROM letras.artistas
    WHERE id_artista IN(' . implode(',', $list) . ')
');

$fotos = $res->fetchAll(\PDO::FETCH_ASSOC);
$res->closeCursor();

// Merge (fazendo)
array_map(function($a, $b) {
    if ($a['id_artista'] == $b['id_artista']){
        return $merge
    }
}, $artistas, $fotos);