有条件的左外联接(位置，顺序)? [英] LEFT OUTER JOIN with conditions (where, order by)?

问题描述

我正在使用MySql(本地)和Postgresql(Heroku)数据库进行Rails3项目.
我需要在左外联接上添加条件，但我不知道该怎么做.

I'm working on a Rails3 project with MySql (local) and Postgresql (Heroku) databases.
I need to put conditions on a LEFT OUTER JOIN but I don't know how.

我有2张桌子TRAININGS和TRAINING_HISTORIES.
我想检索TRAININGS的所有记录，并添加TRAINING_HISTORIES的最后一个有效(也称为完成)的关联记录.

I have 2 tables TRAININGS and TRAINING_HISTORIES.
I want to retrieve all the records of TRAININGS and add the last valid (aka finished) associated record of TRAINING_HISTORIES.

id  name  order_by
5   A     1  
6   B     2  
7   C     3

表TRAINING_HISTORIES

id  training_id finished_at score
43  5           2011-06-06  10
44  5           null        null
45  6           2011-07-07  11
46  6           2011-08-08  14
47  6           null        null
48  6           2011-09-09  18
49  6           null        null
50  7           null        null
51  7           2011-10-10  19

这是我的SQL查询:

Here's my SQL query :

SELECT tc.id, tc.name, tc.order, 
th.id as history_id, th.finished_at, th.score
FROM trainings tc
LEFT OUTER JOIN training_histories th ON th.training_id = tc.id
WHERE tc.id > 4
AND tc.id < 8
GROUP BY tc.id
ORDER BY tc.order_by ASC, tc.id ASC

我得到的结果:

id  name  order history_id  finished_at score
5   A     1     43          2011-06-06  10
6   B     2     45          2011-07-07  11
7   C     3     50          null        null

• 查询检索每个联接的第一个training_history

id  name  order history_id  finished_at score
5   A     1     43          2011-06-06  10
6   B     2     48          2011-09-09  18
7   C     3     51          2011-10-10  19

• 在这种情况下:检索到的是最后完成的training_history ...

任何建议都非常感谢！

谢谢

如果有人可以回答Rails部分，那也可能很棒；-)
如何将SQL查询转换为Rails Active Record查询?

If someone can answer on the Rails part, it could be great too ;-)
How to Convert SQL Query to Rails Active Record Query?

推荐答案

尝试此查询，它将为您提供每种培训以及每种培训的最新培训历史:

Try this Query, it would give you each training and the most recent training history for each one:

SELECT tc.id, tc.name, tc.order, 
th.id as history_id, th.finished_at, th.score
FROM trainings tc
LEFT OUTER JOIN training_histories th ON th.training_id = tc.id 
    and th.id =
    (SELECT th1.id from training_histories th1 where th1.training_id = tc.id
     and th1.finished_at is not null
     order by th1.finished_at desc limit 1)
WHERE tc.id > 4
AND tc.id < 8
GROUP BY tc.id
ORDER BY tc.order_by ASC, tc.id ASC

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