扩展我的评论：
你的模式会更好 id | val_id | val 根据你的数据，你会发现：

+----+--------+------+
| id | val_id | val  |
+----+--------+------+
|  1 |      1 | NULL |
|  1 |      2 | NULL |
|  1 |      3 | a    |
|  1 |      4 | b    |
|  1 |      5 | NULL |
|  1 |      6 | NULL |
|  2 |      1 | NULL |
|  2 |      2 | NULL |
|  2 |      3 | NULL |
|  2 |      4 | b    |
|  2 |      5 | c    |
|  2 |      6 | d    |
+----+--------+------+

这样你就可以回答你的问题：

SELECT <TOP 5> * FROM yourtable ORDER BY id, val_id <LIMIT 5>

哪里 <TOP 5> 以及 <LIMIT 5> 是可互换的，这取决于您使用的rdbms。
要使数据看起来像这样更好的模式，可以使用联合查询：

SELECT id, 1, val1 FROM yourtable
UNION ALL 
SELECT id, 2, val2 FROM yourtable
UNION ALL
SELECT id, 3, val3 FROM yourtable
UNION ALL
SELECT id, 4, val4 FROM yourtable
UNION ALL
SELECT id, 5, val5 FROM yourtable
UNION ALL
SELECT id, 6, val6 FROM yourtable

您可以将其弹出到子查询中以代替 yourtable 在上面的sql中：

SELECT <TOP 5> * 
FROM
    (
        SELECT id, 1, val1 FROM yourtable
        UNION ALL 
        SELECT id, 2, val2 FROM yourtable
        UNION ALL
        SELECT id, 3, val3 FROM yourtable
        UNION ALL
        SELECT id, 4, val4 FROM yourtable
        UNION ALL
        SELECT id, 5, val5 FROM yourtable
        UNION ALL
        SELECT id, 6, val6 FROM yourtable
    ) yourtable 
 ORDER BY id, val_id <LIMIT 5>