MySQL matching phone calls from two tables - mysql

Question

I've been trying to solve an apparently simple problem for a couple of days now, and I can't seem to figure it out flawlessly, no matter which attempt I take.
I've two simple Tables, each with 3 cols: call_date, call_time, caller_id all of them are self-explanatory.
The first table contains inbound calls, the second one contains outbound calls.
After receiving an inbound call, the corresponding outbound call should be made within 30 minutes. So far so good, no problem here, but this is where it gets a little bit tricky:
There can be multiple inbound calls from the same caller_id in the inbound table.
Only calls that have a minimum of 30 Minutes distance to the previous relevant call are regarded as valid calls. So when someone calls four times, let's say at 08:00, at 08:15, at 08:31 and at 09:31, there are three valid calls: 08:00, 08:31 and 09:31. So after each valid call you have to wait 30 minutes again in order for the next call to be valid.
The outbound table can also contain none, one, or multiple calls to the same number, all of which must be matched as closely as possible to the incoming calls.
I tried various approaches to this problem, but all of them are not 100% accurately, always leaving one or more calls unmatched or not marked as valid etc, and my head is starting to spin a lil bit.
Right now I build a huge table from my inbound call table, adding numerous columns for the previous_calltime and next_calltime so I can calculate the minutes between the time stamps. A second approach I took, was to create seperate tables with the first and last calls each day of each number etc, but something always does not add up.
Here's a sample of my data tables, in- and outbound are the same.
+----+-----------+----------+----------+
| id | call_date | call_time|caller_id
+----+-----------+----------+----------+
| 1 | 2013-06-01| 08:00 | 12345
+----+-----------+----------+----------+
| 2 | 2013-06-01| 08:20 | 12345
+----+-----------+----------+----------+
| 3 | 2013-06-01| 08:30 | 12345
+----+-----------+----------+----------+
| 4 | 2013-06-01| 08:32 | 555-999
+----+-----------+----------+----------+
| 5 | 2013-06-01| 08:47 | 555-999
+----+-----------+----------+----------+
Maybe someone can point me in a rough direction which approach I should take.
Any suggestions are appreciated.
P.s. I am trying a 100% MySQL based approach here, but theoretically I could also make use of PHP, just in case someone thinks this is a pain to do in pure SQL and it could be much more easy using PHP.

Answer 1

Initial code to play with.
This is ignoring dates (for now) but should give you a list of the valid calls. Not sure it will be that quick, but it might be an idea to use it to populate a list somewhere of valid calls (or to mark the inbound calls as valid or not).
Using you test data above it gives the results I would expect:-
SELECT Sub1.*,
#valid := if(#PrevCallerId != Sub1.caller_id OR (TIME_TO_SEC(call_time) - #PrevValidCall) >= 1800, "valid", "invalid") AS valid_call,
#PrevValidCall := if(#valid = "valid", TIME_TO_SEC(Sub1.call_time), #PrevValidCall),
#PrevCallerId := Sub1.caller_id
FROM
(
SELECT *
FROM Inbound
ORDER BY caller_id
) Sub1
CROSS JOIN (SELECT #PrevValidCall:= 0, #PrevCallerId := "", #valid := "") Sub2