field in subquery based on age of row instead of "group by" - mysql

Question

I can't seem to get this query right. I have tables like this (simplified):
person: PersonID, ...other stuff...
contact: ContactID, PersonID, ContactDate, ContactTypeID, Description
I want to get a list of all the people who had a contact of a certain type (or types) but none of another type(s) that occurred later. An easy-to-understand example: Checking for records of gifts received without having sent a thank-you card afterward. There might have been other previous thank-you cards sent (pertaining to other gifts), but if the most recent occurrence of a Gift Received (we'll say that's ContactTypeID=12) was not followed by a Thank You Sent (ContactTypeID=11), the PersonID should be in the result set. Another example: A mailing list would be made up of everyone who has opted in (12) without having opted out (11) more recently.
My attempt at a query is this:
SELECT person.PersonID FROM person
INNER JOIN (SELECT PersonID,ContactTypeID,MAX(ContactDate) FROM contact
WHERE ContactTypeID IN (12,11) GROUP BY PersonID) AS seq
ON person.PersonID=seq.PersonID
WHERE seq.ContactTypeID IN (12)`
It seems that the ContactTypeID returned in the subquery is for the last record entered in the table, regardless of which record has the max date. But I can't figure out how to fix it. Sorry if this has been asked before (almost everything has!), but I don't know what terms to search for.

Answer 1

Wow. A system to check who has been good and sent thank yous. I think I would be in your list...
Anyway. Give this a go. The idea is to create two views: the first with personId and the time of the most recently received gift and the second with personId and the most recently sent thanks. Join them together using a left outer join to ensure that people who have never sent a thank you are included and then add in a comparison between the most recently received time and the most recent thanks time to find impolite people:
select g.personId,
g.mostRecentGiftReceivedTime,
t.mostRecentThankYouTime
from
(
select p.personId,
max(ContactDate) as mostRecentGiftReceivedTime
from person p inner join contact c on p.personId = c.personId
where c.ContactTypeId = 12
group by p.personId
) g
left outer join
(
select p.personId,
max(ContactDate) as mostRecentThankYouTime
from person p inner join contact c on p.personId = c.personId
where c.ContactTypeId = 11
group by p.personId
) t on g.personId = t.personId
where t.mostRecentThankYouTime is null
or t.mostRecentThankYouTime < g.mostRecentGiftReceivedTime;
Here is the test data I used:
create table person (PersonID int unsigned not null primary key);
create table contact (
ContactID int unsigned not null primary key,
PersonID int unsigned not null,
ContactDate datetime not null,
ContactTypeId int unsigned not null,
Description varchar(50) default null
);
insert into person values (1);
insert into person values (2);
insert into person values (3);
insert into person values (4);
insert into contact values (1,1,'2013-05-01',12,'Person 1 Got a present');
insert into contact values (2,1,'2013-05-03',11,'Person 1 said "Thanks"');
insert into contact values (3,1,'2013-05-05',12,'Person 1 got another present. Lucky person 1.');
insert into contact values (4,2,'2013-05-01',11,'Person 2 said "Thanks". Not sure what for.');
insert into contact values (5,2,'2013-05-08',12,'Person 2 got a present.');
insert into contact values (6,3,'2013-04-25',12,'Person 3 Got a present');
insert into contact values (7,3,'2013-04-30',11,'Person 3 said "Thanks"');
insert into contact values (8,3,'2013-05-02',12,'Person 3 got another present. Lucky person 3.');
insert into contact values (9,3,'2013-05-05',11,'Person 3 said "Thanks" again.');
insert into contact values (10,4,'2013-04-30',12,'Person 4 got his first present');