从两个表中获取比较日期时间字段的最新数据

Question

我有两个结构相同的表，如下所示：

表：用户

+----+--------+--------------+----------------------+
| ID | Name   |    Password  |    LastUpdateTime    |
+----+--------+--------------+----------------------+
|  1 | abu    |    abu123NEW |  2014-06-04 14:55:06 |
|  2 | john   |      john123 |  2014-06-04 14:58:22 |
|  3 | shane  |     shane123 |  2014-06-04 15:02:06 |
|  4 | marie  |     marie123 |  2014-06-04 15:00:06 |
|  5 | mike   |   mike123NEW |  2014-06-04 15:01:32 |
|  6 | kiron  |  kiron123NEW |  2014-06-04 15:05:46 |
+----+--------+--------------+----------------------+

表：user_k

+----+--------+--------------+----------------------+
| ID | Name   |    Password  |    LastUpdateTime    |
+----+--------+--------------+----------------------+
|  1 | abu    |       abu123 |  2014-06-04 14:53:06 |
|  2 | john   |   john123NEW |  2014-06-04 14:59:48 |
|  3 | shane  |  shane123NEW |  2014-06-04 15:00:06 |
|  4 | marie  |  marie123NEW |  2014-06-04 15:03:17 |
|  5 | mike   |      mike123 |  2014-06-04 15:00:36 |
|  6 | kiron  |     kiron123 |  2014-06-04 15:02:18 |
+----+--------+--------------+----------------------+

现在我需要从这两个表中获取最新数据（通过“LastUpdateTime”），如下所示：

+----+--------+--------------+----------------------+
| ID | Name   |    Password  |    LastUpdateTime    |
+----+--------+--------------+----------------------+
|  1 | abu    |    abu123NEW |  2014-06-04 14:55:06 |
|  2 | john   |   john123NEW |  2014-06-04 14:59:48 |
|  3 | shane  |     shane123 |  2014-06-04 15:02:06 |
|  4 | marie  |  marie123NEW |  2014-06-04 15:03:17 |
|  5 | mike   |   mike123NEW |  2014-06-04 15:01:32 |
|  6 | kiron  |  kiron123NEW |  2014-06-04 15:05:46 |
+----+--------+--------------+----------------------+

我尝试过的：

SELECT uk.* FROM user AS ua, user_k AS uk
WHERE  uk.ID = ua.ID AND uk.LastUpdateTime > ua.LastUpdateTime

UNION

SELECT ua.* FROM user AS ua ,user_k AS uk
WHERE  uk.ID = ua.ID AND uk.LastUpdateTime < ua.LastUpdateTime

我不确定这是否合适。此外，我担心大量数据的查询性能。有没有更好的方法呢？

PS：除了名称和密码之外，两个表中还有十几个字段（列）。我避免使用它们只是为了简化问题。

Answer 1

select
u.id,
u.name,
if(u.LastUpdateTime >= k.LastUpdateTime, u.password, k.password) as password,
greatest(u.LastUpdateTime, k.LastUpdateTime) as LastUpdateTime
from
user u
inner join user_k k on u.id = k.id

• 查看它在 sqlfiddle 中的实时工作

P.S.：对于 id 3，较新的密码实际上是来自用户表的密码，而不是 user_k。您想要的结果集需要此调整。