【mysql】[error]group_concatによるsql構文エラー

$sql_id = "select group_concat(`table_a_id`) from `table_b`";
$id_str = $db->execute($sql_id);

$sql = "select * from `table_a` where `id` in ( {$id_str} ) and `is_effect` = 1`";
$res = $db->execute($sql);

/*  
    error:You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right 
    syntax to use near ') and `is_effect` = 1'
*/
//   sql： select * from `table_a` where `id` in ( 1,2,3,4, ) and `is_effect` = 1`