MySQL-在JOIN中选择给定日期之前的最新记录

Question

忍受我，我是SQL的新手，在解释这个问题时遇到了麻烦...

我有两个类似于以下的表：

test1
+------+----------+----------+-----------+
|  id  |  serial  |  t1data  |  t1date   |
+------+----------+----------+-----------+
|  1   |    a     |   ...    |  6/02/12  |
|  2   |    a     |   ...    |  6/04/12  |
|  3   |    b     |   ...    |  6/06/12  |
|  4   |    a     |   ...    |  6/08/12  |
+------+----------+----------+-----------+

test2
+------+----------+----------+-----------+
|  id  |  serial  |  t2data  |  t2date   |
+------+----------+----------+-----------+
|  1   |    a     |   ...    |  6/05/12  |
|  2   |    b     |   ...    |  6/07/12  |
|  3   |    b     |   ...    |  6/08/12  |
|  4   |    a     |   ...    |  6/09/12  |
+------+----------+----------+-----------+

我想创建一个查询联接两个表，以便在每个记录test2对应于最新test1该日期之前发生的记录test2 。 任何给定的序列可以在任一表中具有多个记录。

使用上面的示例，结果应如下所示：

+----------+----------+-----------+----------+-----------+
|  serial  |  t2data  |  t2date   |  t1data  |  t1date   |
+----------+----------+-----------+----------+-----------+
|    a     |   ...    |  6/05/12  |   ...    |  6/04/12  |
|    b     |   ...    |  6/07/12  |   ...    |  6/06/12  |
|    b     |   ...    |  6/08/12  |   ...    |  6/06/12  |
|    a     |   ...    |  6/09/12  |   ...    |  6/08/12  |
+----------+----------+-----------+----------+-----------+

这是我尝试过的查询，但不起作用。

SELECT 
    t2.serial, t2.t2data, t2.t2date, t1a.t1data, t1a.t1date
FROM 
    test2 t2
JOIN 
    test1 t1a ON (t1a.serial = t2.serial AND t1a.t1date > t2.t2date)
LEFT JOIN 
    test1 t1b ON (t1a.serial = t1b.serial AND t1a.t1date < t1b.t1date)
WHERE 
    t1b.serial IS NULL

我可能会全都错了，但是我该怎么做才能完成这项工作？

Answer 1

我不确定这是否是最好的方法，但是它在起作用：

SELECT t2.serial, t2.t2data, t1.t1data, t1.t1date
FROM
  (SELECT t2.serial, t2.t2data, t2.t2date,
         (SELECT max(t1date)
            FROM test1 t1
           WHERE t1.serial = t2.serial
             AND t1.t1date <= t2.t2date) AS t1date
  FROM test2 t2) t2
  LEFT OUTER JOIN
   test1 t1 ON t2.serial = t1.serial AND t2.t1date = t1.t1date;

http://www.sqlfiddle.com/#!2/215c263/2