Question

我正在尝试将table2中的最后一个水果计数追加到table1上的查询。

table1                              table2                   
+--------+--------+-------+-----+   +--------+------+-------+
| fruit  | season | carbs | fat |   | fruit  | date | count |
+--------+------+---------+-----+   +--------+------+-------+
| apple  | summer |   21  |  0  |   | apple  | 2015 |  700  |
| banana | all yr |   27  |  0  |   | apple  | 2014 |  500  |
+--------+--------+-------+-----+   | banana | 2014 |  200  |
                                    | banana | 2013 |  300  |
                                    +--------+------+-------+

goal:
+--------+--------+-------+-----+------------+
| fruit  | season | carbs | fat | last_count |
+--------+------+---------+-----+------------+
| apple  | summer |   21  |  0  |     700    |
| banana | all yr |   27  |  0  |     200    |
+--------+--------+-------+-----+------------+

我能想到的就是这样（不工作）：

SELECT t1.*, t2.count AS last_count FROM table1 AS t1
INNER JOIN 
(SELECT fruit, count FROM table2 ORDER BY date DESC) t2 ON t1.fruit = t2.fruit

此外，我应该能够根据table1中的值过滤结果（例如WHERE t1.carbs < 25）

Answer 1

解决此问题的一种方法是将table1加入table2两次。第一个连接是具有计数的完整table2，第二个连接是table2的子查询，其中仅包含每个水果的最新水果记录。

SELECT t1.*, t2.count AS last_count
FROM table1 t1
INNER JOIN table2 t2
    ON t1.fruit = t2.fruit
INNER JOIN
(
    SELECT fruit, MAX(date) AS date
    FROM table2
    GROUP BY fruit
) t3
    ON t2.fruit = t3.fruit AND
       t2.date  = t3.date

子查询来自不同表的最后日期/值

1 个答案: