How to pair date rows in mysql - mysql

Question

How to pair rows with imperfecty in mysql? Imperfect, meaning the rows are not exactly paired as in and out. Sometimes, there are two in with one out or no out at all or two or more out with one in or no in at all.
The table look like this:
| id | action | date |
+----+--------+---------------------+
| 1 | in | 19.1.2012 15:41:52 |
| 1 | out | 19.1.2012 15:55:52 |
| 2 | in | 19.2.2012 15:55:52 |
| 2 | out | 19.2.2012 17:55:53 |
| 1 | in | 19.3.2012 15:55:54 |
| 1 | in | 19.3.2012 17:55:55 |
| 1 | out | 19.3.2012 19:55:56 |
| 3 | in | 19.4.2012 15:55:57 |
| 3 | out | 19.4.2012 17:55:58 |
| 3 | out | 19.4.2012 19:55:59 |
+----+--------+---------------------+
The desired result is like this:
+----+--------+---------------------+
| id | action | date |
+----+--------+---------------------+
| 1 | in | 19.1.2012 15:41:52 |
| 1 | out | 19.1.2012 15:55:52 |
| 2 | in | 19.2.2012 15:55:52 |
| 2 | out | 19.2.2012 17:55:53 |
| 1 | in | 19.3.2012 17:55:55 |
| 1 | out | 19.3.2012 19:55:56 |
| 3 | in | 19.4.2012 15:55:57 |
| 3 | out | 19.4.2012 17:55:58 |
+----+--------+---------------------+
This is the best desireed result
+----+---------------------+---------------------+
| id | date_in | date_out |
+----+---------------------+---------------------+
| 1 | 19.1.2012 15:41:52 | 19.1.2012 15:55:52 |
| 2 | 19.2.2012 15:55:52 | 19.2.2012 17:55:53 |
| 1 | 19.3.2012 17:55:55 | 19.3.2012 19:55:56 |
| 3 | 19.4.2012 15:55:57 | 19.4.2012 17:55:58 |
+----+---------------------+---------------------+
Here is a code, but it yields different result, can anyone figure out where the error is?
enter code here SELECT c.e_id
, CAST(c.in_time AS DATETIME) AS in_time
, c.out_time
FROM (
SELECT IF(#prev_id = d.id,#in_time,#in_time:=NULL) AS reset_in_time
, #in_time := IF(d.action = 'in',d.date,#in_time) AS in_time
, IF(d.action = 'out',d.date,NULL) AS out_time
, #prev_id := d.id AS id
FROM (
SELECT id, date_, action
FROM e
JOIN (SELECT #prev_id := NULL, #in_time := NULL) f
ORDER BY id, date, action
) d
) c
WHERE c.out_time IS NOT NULL
ORDER BY c.out_time enter code here

Answer 1

This selects an in event into a, an out event into b and uses a LEFT JOIN to eliminate the row if there exists an event c for the same id between them; that will simply get all in/out times that don't have an extra in or out between them.
SELECT a.id, a.date date_in, b.date date_out
FROM mytable a
JOIN mytable b
ON a.id = b.id AND a.date < b.date
LEFT JOIN mytable c
ON a.id = c.id AND c.date < b.date AND c.date > a.date
WHERE a.action = 'in' AND b.action = 'out' AND c.action IS NULL
ORDER BY a.date;
An SQLfiddle to test with.