MySQL - condition on the joined row from from right table - mysql

Question

I have two tables:
mysql> select * from orders;
+------+---------------------+------------+---------+
| id | created_at | foreign_id | data |
+------+---------------------+------------+---------+
| 1 | 2010-10-10 10:10:10 | 3 | order 1 |
| 4 | 2010-10-10 00:00:00 | 6 | order 4 |
| 5 | 2010-10-10 00:00:00 | 7 | order 5 |
+------+---------------------+------------+---------+
mysql> select * from activities;
+------+---------------------+------------+------+
| id | created_at | foreign_id | verb |
+------+---------------------+------------+------+
| 1 | 2010-10-10 10:10:10 | 3 | get |
| 2 | 2010-10-10 10:10:15 | 3 | set |
| 3 | 2010-10-10 10:10:20 | 3 | put |
| 4 | 2010-10-10 00:00:00 | 6 | get |
| 5 | 2010-10-11 00:00:00 | 6 | set |
| 6 | 2010-10-12 00:00:00 | 6 | put |
+------+---------------------+------------+------+
Now I need to join activities with orders on foreign_id column: select only one activity (if exists) for every order such that ABS(TIMESTAMPDIFF(SECOND, orders.created_at, activities.created_at)) is minimal. E.g. the order and the activity were created approximately at the same time.
+----------+---------+---------------------+-------------+------+---------------------+
| order_id | data | order_created_at | activity_id | verb | activity_created_at |
+----------+---------+---------------------+-------------+------+---------------------+
| 1 | order 1 | 2010-10-10 10:10:10 | 1 | get | 2010-10-10 10:10:10 |
| 4 | order 4 | 2010-10-10 00:00:00 | 4 | get | 2010-10-10 00:00:00 |
| 5 | order 5 | 2010-10-10 00:00:00 | NULL | NULL | NULL |
+----------+---------+---------------------+-------------+------+---------------------+
The following query produces set of rows that includes the desired rows. If GROUP BY statement is included then it's not possible to control which row from activities is joined.
SELECT o.id AS order_id
, o.data AS data
, o.created_at AS order_created_at
, a.id AS activity_id
, a.verb AS verb
, a.created_at AS activity_created_at
FROM orders AS o
LEFT JOIN activities AS a ON a.foreign_id = o.foreign_id;
Is it possible to write such a query? Ideally I'd like to avoid using group by because this section is a part of larger reporting querty.

Answer 1

Because both tables reference some mysterious foreign key there's potential for errors with the query below, but it may give you a principle which you can adapt for your purposes...
DROP TABLE IF EXISTS orders;
CREATE TABLE orders
(id INT NOT NULL PRIMARY KEY
,created_at DATETIME NOT NULL
,foreign_id INT NOT NULL
,data VARCHAR(20) NOT NULL
);
INSERT INTO orders VALUES
(1 ,'2010-10-10 10:10:10',3 ,'order 1'),
(4 ,'2010-10-10 00:00:00',6 ,'order 4'),
(5 ,'2010-10-10 00:00:00',7 ,'order 5');
DROP TABLE IF EXISTS activities;
CREATE TABLE activities
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,created_at DATETIME NOT NULL
,foreign_id INT NOT NULL
,verb VARCHAR(20) NOT NULL
);
INSERT INTO activities VALUES
(1,'2010-10-10 10:10:10',3,'get'),
(2,'2010-10-10 10:10:15',3,'set'),
(3,'2010-10-10 10:10:20',3,'put'),
(4,'2010-10-10 00:00:00',6,'get'),
(5,'2010-10-11 00:00:00',6,'set'),
(6,'2010-10-12 00:00:00',6,'put');
SELECT o.id order_id
, o.data
, o.created_at order_created_at
, a.id activity_id
, a.verb
, a.created_at activity_created_at
FROM activities a
JOIN orders o
ON o.foreign_id = a.foreign_id
JOIN
( SELECT a.foreign_id
, MIN(ABS(TIMEDIFF(a.created_at,o.created_at))) x
FROM activities a
JOIN orders o
ON o.foreign_id = a.foreign_id
GROUP
BY a.foreign_id
) m
ON m.foreign_id = a.foreign_id
AND m.x = ABS(TIMEDIFF(a.created_at,o.created_at))
UNION DISTINCT
SELECT o.id
, o.data
, o.created_at
, a.id
, a.verb
, a.created_at
FROM orders o
LEFT
JOIN activities a
ON a.foreign_id = o.foreign_id
WHERE a.foreign_id IS NULL;
;
+----------+---------+---------------------+-------------+------+---------------------+
| order_id | data | order_created_at | activity_id | verb | activity_created_at |
+----------+---------+---------------------+-------------+------+---------------------+
| 1 | order 1 | 2010-10-10 10:10:10 | 1 | get | 2010-10-10 10:10:10 |
| 4 | order 4 | 2010-10-10 00:00:00 | 4 | get | 2010-10-10 00:00:00 |
| 5 | order 5 | 2010-10-10 00:00:00 | NULL | NULL | NULL |
+----------+---------+---------------------+-------------+------+---------------------+