我有一个SQL查询的问题,返回的结果集不是我所期望的。
我有这三个表,我想把它们联系起来。
events_detail
__________________
| ID | start_date |
| 1 | 2012-08-09 |
| 2 | 2013-02-13 |
| 3 | 2012-12-12 |
| 4 | 2013-01-21 |
| 5 | 2012-12-25 |
-------------------
,其中ID是主键
events_category_relationship
__________________________
| ID | event_id | cat_id |
| 1 | 1 | 1 |
| 2 | 2 | 4 |
| 3 | 3 | 2 |
| 4 | 4 | 2 |
| 5 | 5 | 3 |
--------------------------
其中ID为主键
events_category_detail
__________________________________
| ID | name | description |
| 1 | Europe | Kings and castles! |
| 2 | USA | Freedoms |
| 3 | China | Made in China |
| 4 | UK | Big Brother |
------------------------------------
其中ID为主键
我需要做的是从每个类别中抓取1个事件,并按最早出现的日期排序。所以我应该在我的结果中期待的是下面的
结果集________________________________________________________________
| e_id | start_date | c_id | category_name | category_desc |
| 1 | 2012-08-09 | 1 | Europe | Kings and castles! |
| 3 | 2012-12-12 | 2 | USA | Freedoms |
| 5 | 2012-12-25 | 3 | China | Made in China |
| 2 | 2013-02-13 | 4 | UK | Big Brother |
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我尝试的SQL查询看起来像这样
SELECT e.id, e.start_date, c.category_name, c.category_desc
FROM events_detail e
JOIN events_category_relationship r ON r.event_id = e.id
JOIN events_category_detail c ON c.id = r.cat_id
ORDER BY date(e.start_date)
这只是连接3个表并按日期顺序返回结果。我所坚持的是使每个类别中只有一个像上面期望的结果集一样显示。我试过使用DISTINCT c.category_name和GROUP BY c.category_name,但它们都不起作用。
您将需要使用子查询来获取每个name
和description
的min(start_date)
。然后,您将使用此结果连接回events_details
表,以获得与子查询中的数据相关联的id
:
SELECT e.id,
d.start_date,
d.name,
d.description
FROM events_detail e
INNER JOIN
(
select min(e.start_date) Start_date,
c.name,
c.description
from events_detail e
INNER JOIN events_category_relationship r
ON r.event_id = e.id
INNER JOIN events_category_detail c
ON c.id = r.cat_id
group by c.name, c.description
) d
on e.start_date = d.Start_date
ORDER BY date(e.start_date)
参见SQL Fiddle with Demo