Find next date value in the column - sql

Question

I have a large table with the following columns and sample values:
ID Ser Reg Date
1 12345 001 1/3/2011
1 12345 001 2/2/2011
1 12345 002 1/3/2011
1 12345 002 2/2/2011
2 23456 001 1/3/2011
2 23456 001 2/7/2011
2 23456 001 3/5/2011
I tried this query from a previous post SQL - Select next date query - but did not get the desired results:
SELECT
mytable.id,
mytable.date,
(
SELECT
MIN(mytablemin.date)
FROM mytable AS mytablemin
WHERE mytablemin.date > mytable.date
) AS NextDate
FROM mytable
This is what I am trying to accomplish:
ID Ser Reg curr_Date prev_Date
1 12345 001 2/2/2011 1/3/2011
1 12345 002 2/2/2011 1/3/2011
2 23456 001 2/7/2011 1/5/2011
2 23456 001 3/5/2011 2/7/2011
I would appreciate any help with this task.

Answer 1

if you are using oracle database (as you have not mentioned then I can assume anything)
then you can use lead and lag function/command for this ..
select id,ser, reg, curr_date ,prev_date
from
(
select id,ser, reg, ser, date curr_date
LEAD(date, 1, 0) OVER (PARTITION BY id,ser, reg, curr_date ORDER BY date DESC NULLS LAST) prev_date,
)
where prev_date is not null;

Answer 2

There was a condition missing from correlated subquery joining mytablemin copy of mytable table with mytable. Also you would eliminate records which do not have NextDate - but this might give incorrect results in case when only one record in group (Id, Ser, Reg) exists by eliminating it from result set.
select * from
(
SELECT
mytable.id,
mytable.date,
(
SELECT
MIN(mytablemin.date)
FROM mytable AS mytablemin
WHERE mytablemin.date > mytable.date
and mytablemin.id = mytable.id
and mytablemin.Ser = mytable.Ser
and mytablemin.Reg = mytable.Reg
) AS NextDate
FROM mytable
) a
where a.NextDate is not null
And here is version using derived table with aggregation:
SELECT
mytable.id,
mytable.date,
mytablemin.minDate
FROM mytable
inner join
(
SELECT mytablemin.id,
mytablemin.Ser,
mytablemin.Reg,
MIN(mytablemin.date) minDate
FROM mytable AS mytablemin
group by mytablemin.id,
mytablemin.Ser,
mytablemin.Reg
having MIN(mytablemin.date) is not null
) AS mytablemin
on mytablemin.id = mytable.id
and mytablemin.Ser = mytable.Ser
and mytablemin.Reg = mytable.Reg