Question

我有2个表，我的目标是只有一个SELECT查询返回项目名称，与该项目相关的子项目数以及最近添加的子项目是什么。

表-A

+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| item_ID   | int(11)      | NO   | PRI | NULL    | auto_increment |
| item_name | varchar(300) | NO   |     | NULL    |                |
+-----------+--------------+------+-----+---------+----------------+

表-B

+------------------+--------------+------+-----+---------+----------------+
| Field            | Type         | Null | Key | Default | Extra          |
+------------------+--------------+------+-----+---------+----------------+
| subitem_ID       | int(12)      | NO   | PRI | NULL    | auto_increment |
| subitem_relation | int(12)      | NO   |     | NULL    |                |
| subitem_name     | varchar(300) | NO   |     | NULL    |                |
| subitem_date     | datetime     | NO   |     | NULL    |                |
+------------------+--------------+------+-----+---------+----------------+

这是我在table_b中的数据：

+------------+------------------+--------------+---------------------+
| subitem_ID | subitem_relation | subitem_name | subitem_date        |
+------------+------------------+--------------+---------------------+
|          1 |                1 | subitem a    | 2014-06-09 10:51:20 |
|          2 |                1 | subitem b    | 2014-06-10 05:21:52 |
|          3 |                2 | subitem c    | 2014-06-16 05:12:10 |
|          4 |                1 | subitem d    | 2014-06-18 06:42:14 |
|          5 |                2 | subitem e    | 2014-06-23 19:33:08 |
|          6 |                2 | subitem f    | 2014-07-02 12:30:25 |
|          7 |                2 | subitem g    | 2014-07-04 21:52:40 |
+------------+------------------+--------------+---------------------+

这是我目前的查询：

SELECT 
    a.item_name,
    COUNT(b.subitem_ID) as subitem_no,
    b.subitem_name as most_recent_subitem
FROM
    table_a as a
INNER JOIN 
    table_b as b
ON 
    a.item_ID = b.subitem_relation
GROUP BY 
    b.subitem_relation;

这就是结果：

+-------------+------------+---------------------+
| item_name   | subitem_no | most_recent_subitem |
+-------------+------------+---------------------+
| first item  |          3 | subitem a           |
| second item |          4 | subitem c           |
+-------------+------------+---------------------+

这将返回正确数量的子项，但它返回第一个相关子项而不是最新项。我需要以某种方式将table_b中的数据排序到ORDER BY subitem_date DESC，但不知道如何实现它。

我要找的结果是：

+-------------+------------+---------------------+
| item_name   | subitem_no | most_recent_subitem |
+-------------+------------+---------------------+
| first item  |          3 | subitem d           |
| second item |          4 | subitem g           |
+-------------+------------+---------------------+

Answer 1

我会使用group_concat() / substring_index()技巧：

执行此操作

SELECT a.item_name, COUNT(b.subitem_ID) as subitem_no,
       substring_index(group_cooncat(b.subitem_name order by subitem_date desc
                                    ), ',', 1) as most_recent_subitem
FROM table_a a INNER JOIN 
     table_b b
     ON  a.item_ID = b.subitem_relation
GROUP BY a.item_ID;

MYSQL查询：订购INNER JOIN数据

1 个答案: