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    <p>How does the following query perform? It works by first finding the northmost Y cordinate in the set of airports visited. Then an identical query is performed which is filtered by the Y coordinate in the previous query. The final step is to find the airport.</p> <pre><code>drop table airports; drop table flights; create table airports( apid int not null ,apname varchar(50) not null ,x int not null ,y int not null ,primary key(apid) ,unique(apname) ); create table flights( flight_id int not null auto_increment ,src_apid int not null ,dst_apid int not null ,user_id varchar(20) not null ,foreign key(src_apid) references airports(apid) ,foreign key(dst_apid) references airports(apid) ,primary key(flight_id) ,index(user_id) ); insert into airports(apid, apname, x, y) values(1, 'Northpole Civilian', 50, 100); insert into airports(apid, apname, x, y) values(2, 'Northpole Military', 50, 100); insert into airports(apid, apname, x, y) values(3, 'Transit point', 50, 50); insert into airports(apid, apname, x, y) values(4, 'Southpole Civilian', 50, 0); insert into airports(apid, apname, x, y) values(5, 'Southpole Military', 50, 0); insert into flights(src_apid, dst_apid, user_id) values(4, 3, 'Family guy'); insert into flights(src_apid, dst_apid, user_id) values(3, 1, 'Family guy'); insert into flights(src_apid, dst_apid, user_id) values(5, 3, 'Mr Bazooka'); insert into flights(src_apid, dst_apid, user_id) values(3, 2, 'Mr Bazooka'); select airports.apid ,airports.apname ,airports.x ,airports.y from (select max(a.y) as y from flights f join airports a on (a.apid = f.src_apid or a.apid = f.dst_apid) where f.user_id = 'Family guy' ) as northmost join (select a.apid ,a.y from flights f join airports a on (a.apid = f.src_apid or a.apid = f.dst_apid) where f.user_id = 'Family guy' ) as userflights on(northmost.y = userflights.y) join airports on(userflights.apid = airports.apid); </code></pre> <p><strong>Edit. Alternative query that may be less confusing to the optimizer</strong> </p> <pre><code>select airports.* from (select case when s.y &gt; d.y then s.apid else d.apid end as apid ,case when s.y &gt; d.y then s.y else d.y end as northmost from flights f join airports s on(f.src_apid = s.apid) join airports d on(f.dst_apid = d.apid) where f.user_id = 'Family guy' order by northmost desc limit 1 ) as user_flights join airports on(airports.apid = user_flights.apid); </code></pre>
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    1. PONested query to find details in table B for maximum value in table A
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    1. USRonnis
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    1. COOh, I didn't see you added example data while I was writing this. Would you like me use your example instead?
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    2. CODing ding ding, I think we have a winner! Gets the right results and is super-fast to boot. The only somewhat scary thing is the temporary table and that DESCRIBE says a filesort is required, but I'll try out a few pathological cases and report back.
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    3. COSurvey says: a highly respectable 0.23s for >4000 flights and a rather tolerable 3.15s for 40,000 flights. The bounty is yours!
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