Question

假设我有某种SELECT查询，它向我显示了10行。

有没有办法可以更改此查询，以便每次显示两行，总共给出20个？

此外，可以在SELECT中添加一些任意标识符以区分每对行吗？

Answer 1

这应该有效：

select a, b, c, 1 as pair from myTable
union all
select a, b, c, 2 as pair from myTable

Answer 2

是的，你可以使用null这样做。跑吧。

UNION ALL

Answer 3

详细说明@ cdaiga的答案并在每行中包含一个唯一标识符......

SET @i := 0;
SELECT <your columns>, @i := @i + 1 as num from ...
UNION ALL
SELECT <your columns>, @i := @i + 1 as num from ...

然后，您将拥有一个带有连续序列号的num列。这是一个例子：fiddle。

Answer 4

使用任意标识符选择两次列，然后使用union all

取消/转换为行

例如给定

MariaDB [sandbox]> select fb_uid,id,username from users;
+--------+----+----------+
| fb_uid | id | username |
+--------+----+----------+
|      1 |  1 | John     |
|      2 |  2 | Jane     |
|      3 |  3 | Ali      |
|      6 |  6 | Bruce    |
|      7 |  7 | Martha   |
|      8 |  8 | Sidney   |
|     10 | 10 | charlie  |
|     12 | 12 | Elisa    |
|     14 | 14 | Samantha |
|     15 | 15 | Hannah   |
|     16 | 16 | Hannah   |
|     17 | 17 | Kevin    |
|     18 | 18 | Kevin    |
|     19 | 19 | Ruth     |
+--------+----+----------+
14 rows in set (0.00 sec)

select fb_uid,id1 id,uname1 username
from
(
select fb_uid,id id1,username uname1, id id2, username uname2 from users u
) u1
union all
select fb_uid,id2,uname2
from
(select fb_uid,id id1,username uname1, id id2, username uname2 from users u
) u2
order by id

结果

+--------+----+----------+
| fb_uid | id | username |
+--------+----+----------+
|      1 |  1 | John     |
|      1 |  1 | John     |
|      2 |  2 | Jane     |
|      2 |  2 | Jane     |
|      3 |  3 | Ali      |
|      3 |  3 | Ali      |
|      6 |  6 | Bruce    |
|      6 |  6 | Bruce    |
|      7 |  7 | Martha   |
|      7 |  7 | Martha   |
|      8 |  8 | Sidney   |
|      8 |  8 | Sidney   |
|     10 | 10 | charlie  |
|     10 | 10 | charlie  |
|     12 | 12 | Elisa    |
|     12 | 12 | Elisa    |
|     14 | 14 | Samantha |
|     14 | 14 | Samantha |
|     15 | 15 | Hannah   |
|     15 | 15 | Hannah   |
|     16 | 16 | Hannah   |
|     16 | 16 | Hannah   |
|     17 | 17 | Kevin    |
|     17 | 17 | Kevin    |
|     18 | 18 | Kevin    |
|     18 | 18 | Kevin    |
|     19 | 19 | Ruth     |
|     19 | 19 | Ruth     |
+--------+----+----------+
28 rows in set (0.00 sec)

如何获取查询以显示每条记录的双行？

4 个答案: