MYSQL查询:对内部连接数据进行排序
我有两个表,我的目标是有一个单一的SELECT查询,返回项目名称、有多少子项目与该项目相关以及最近添加的子项目是什么 表aMYSQL查询:对内部连接数据进行排序,mysql,Mysql,我有两个表,我的目标是有一个单一的SELECT查询,返回项目名称、有多少子项目与该项目相关以及最近添加的子项目是什么 表a +-----------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-----------+--------------+------+-----+---------
+-----------+--------------+------+-----+---------+----------------+
| 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 | |
+------------------+--------------+------+-----+---------+----------------+
这是我在表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 |
+-------------+------------+---------------------+
它返回正确数量的子项,但返回的是第一个相关子项,而不是最近的子项。我需要以某种方式按照子项_datedesc对表_b中的数据进行排序,但不确定如何实现这一点
我想要的结果是:
+-------------+------------+---------------------+
| item_name | subitem_no | most_recent_subitem |
+-------------+------------+---------------------+
| first item | 3 | subitem d |
| second item | 4 | subitem g |
+-------------+------------+---------------------+
我会用
group\u concat()
/substring\u index()
技巧做到这一点:
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;
真正伟大的功能里面的功能。。!