Comments on: Question 80: How do you read between the rows?

By: plogi

plogi — Thu, 15 Jul 2010 16:25:06 +0000

Aaaaand still a subquery solution:
select avg(unix_timestamp(v1.sold_ts)-unix_timestamp(v2.sold_ts)) as avg_diff from vending_log v1, vending_log v2 where v2.sold_ts = (select sold_ts from vending_log where sold_ts < v1.sold_ts order by 1 desc limit 1 );

By: plogi

plogi — Thu, 15 Jul 2010 10:39:47 +0000

Here is still a variation on the subquery solution…
Instead of selecting the max(), you can also just select 1 timestamp and limit by 1.
I’m not fond of “limit 1″ for anything else than testing/debugging, but in this case it is faster than the “select max()” solution.

select avg(vv1.diff) from (select *,unix_timestamp(v1.sold_ts) - unix_timestamp((select sold_ts from vending_log where sold_ts < v1 .sold_ts order by sold_ts desc limit 1)) as diff from vending_log v1) vv1;

By: plogi

plogi — Fri, 09 Jul 2010 21:26:52 +0000
Mysql does not have OLAP extensions or windowing functions at this point. So it is not that easy to refer to the previous row. Below is one way of solving the problem: With a subquery to retrieve the previous row. It works quite fine and is easy to read, but it is slow.
select avg(vv1.diff) from (select *,unix_timestamp(v1.sold_ts) - unix_timestamp((select max(sold_ts) from vending_log where sold_ts < v1 .sold_ts)) as diff from vending_log v1) vv1;
The next one is fast, but relies on user variables.
set @a:=null; set @b:=null; select avg(vv1.diff) from (select @b:=@a,@a:=sold_ts, unix_timestamp(sold_ts) - unix_timestamp(@b) as diff from vending_log order by sold_ts) vv1;