SQL with multiple ROW_NUMBER or RANK - sql

Question

I have the need to do multiple joins with the same table, between (eg) Person and PersonEvents. There are multiple events for each person (0 or more). I need to create a VIEW that selects each person with certain columns from their most recent event, plus columns from the next-most-recent event.
Person data:
Id Name
1 Iain
2 Fred
3 Mary
4 Foo
5 Bar
PersonEvents data:
PersonId DateStarted ReasonForLeaving
1 2011-03-12 00:00:00.000 sick
1 2013-02-12 00:00:00.000 NULL
1 2012-04-12 00:00:00.000 holiday
2 2011-05-12 00:00:00.000 new baby
2 2013-06-12 00:00:00.000 NULL
2 2012-07-12 00:00:00.000 had enough
3 2011-08-12 00:00:00.000 pregnant
3 2013-09-12 00:00:00.000 NULL
4 2012-10-12 00:00:00.000 NULL
An output sample would be:
Id Name MemberSince ReasonForChange
1 Iain 2011-03-12 00:00:00.000 holiday
4 Foo 2012-10-12 00:00:00.000 NULL
...
The "old way" used a top 1 join or sub-select statement:
SELECT p.*,
(
SELECT TOP 1 DateStarted
FROM PersonEvents e
WHERE e.PersonId = p.Id
ORDER BY DateFoo DESC
) As MemberSince
FROM Person p
....
However if you need multiple columns from this Join, (eg Date, Comment, and maybe further ids), then you need to do multiple sub-select statements, which is expensive.
So the question is: How do you get multiple columns from a join using the row number for the most recent, and previous events?

Answer 1

The most straight forward (ie. readable SQL) answer that I have come up with uses WITH and ROW_NUMBER.
First, make a ROW_NUMBER query that orders the events and gives a number to each event unique to that PersonId:
SELECT *,
ROW_NUMBER() OVER (PARTITION BY PersonId ORDER BY DateStarted DESC) AS EventOrder
FROM PersonEvents
Results:
PersonId DateStarted ReasonForLeaving EventOrder
1 2013-02-12 00:00:00.000 NULL 1
1 2012-04-12 00:00:00.000 holiday 2
1 2011-03-12 00:00:00.000 sick 3
2 2013-06-12 00:00:00.000 NULL 1
2 2012-07-12 00:00:00.000 had enough 2
2 2011-05-12 00:00:00.000 new baby 3
3 2013-09-12 00:00:00.000 NULL 1
3 2011-08-12 00:00:00.000 pregnant 2
4 2012-10-12 00:00:00.000 NULL 1
Now, the "first" event (in my case the most recent) for every person contains the date that the change was made (real-life example: this is student enrolment history data across multiple schools, containing School ID and lots of other guff). The "Second" event for every person contains the previous event and reason for leaving. To add it together:
WITH SortedEvents AS (
SELECT *,
ROW_NUMBER() OVER (PARTITION BY PersonId ORDER BY ReasonForLeaving DESC) AS EventOrder
FROM PersonEvents
)
SELECT p.*, MostRecent.DateStarted AS MemberSince, NextRecent.ReasonForLeaving AS ReasonForChange
FROM Person p
LEFT OUTER JOIN SortedEvents AS MostRecent ON p.Id = MostRecent.PersonId AND MostRecent.EventOrder = 1
LEFT OUTER JOIN SortedEvents AS NextRecent ON p.Id = NextRecent.PersonId AND NextRecent.EventOrder = 2
which provides the nicely formatted output:
Id Name MemberSince ReasonForChange
1 Iain 2013-02-12 00:00:00.000 holiday
2 Fred 2013-06-12 00:00:00.000 had enough
3 Mary 2013-09-12 00:00:00.000 pregnant
4 Foo 2012-10-12 00:00:00.000 NULL
5 Bar NULL NULL
in reality you could pick multiple columns from any row number. The real life example (again, student enrolment history) picks:
From the master student table:
student id
name
DOB, etc
From the Enrolment History table as "current enrolment"
School id
various enrolment status info
date started
From the Enrolment History table as "previous enrolment"
reason for leaving
This method is quite efficient with about 150k students and their respective history.
complete SQL for my tests:
CREATE TABLE Person
(
Id INT NOT NULL,
Name VARCHAR(50)
)
GO
CREATE TABLE PersonEvents
(
PersonId INT NOT NULL,
DateStarted DATETIME NOT NULL,
ReasonForLeaving VARCHAR(50)
)
GO
INSERT INTO Person
SELECT 1, 'Iain' UNION ALL
SELECT 2, 'Fred' UNION ALL
SELECT 3, 'Mary' UNION ALL
SELECT 4, 'Foo' UNION ALL
SELECT 5, 'Bar'
GO
INSERT INTO PersonEvents
SELECT 1, '20110312', 'sick' UNION ALL
SELECT 1, '20130212', NULL UNION ALL
SELECT 1, '20120412', 'holiday' UNION ALL
SELECT 2, '20110512', 'new baby' UNION ALL
SELECT 2, '20130612', NULL UNION ALL
SELECT 2, '20120712', 'had enough' UNION ALL
SELECT 3, '20110812', 'pregnant' UNION ALL
SELECT 3, '20130912', NULL UNION ALL
SELECT 4, '20121012', NULL
GO
--SELECT *
--FROM Person
--SELECT *
--FROM PersonEvents
--GO
WITH SortedEvents AS (
SELECT *,
ROW_NUMBER() OVER (PARTITION BY PersonId ORDER BY DateStarted DESC) AS EventOrder
FROM PersonEvents
)
SELECT p.*, MostRecent.DateStarted AS MemberSince, NextRecent.ReasonForLeaving AS ReasonForChange
FROM Person p
LEFT OUTER JOIN SortedEvents AS MostRecent ON p.Id = MostRecent.PersonId AND MostRecent.EventOrder = 1
LEFT OUTER JOIN SortedEvents AS NextRecent ON p.Id = NextRecent.PersonId AND NextRecent.EventOrder = 2
GO
SELECT p.*,
(
SELECT TOP 1 DateStarted
FROM PersonEvents pe
WHERE pe.PersonId = p.Id
ORDER BY DateStarted DESC
) AS MemberSince,
'unknown' AS ReasonForChange
FROM Person p
GO
DROP TABLE Person
DROP TABLE PersonEvents
GO

Answer 2

For the last event and previous event date:
SELECT ID,NAME,NextToMostEventDate,ReasonForLeaving
FROM PersonEvents pe
INNER JOIN(
SELECT pe1.PersonId,TheMostEventDate,NextToMostEventDate=MAX(pe1.DateStarted)
FROM PersonEvents pe1
INNER JOIN(
SELECT PersonId,TheMostEventDate=MAX(DateStarted)
FROM PersonEvents
GROUP BY PersonId
) pe2
ON pe2.PersonId=pe1.PersonId
WHERE DateStarted<TheMostEventDate
GROUP BY pe1.PersonId,TheMostEventDate
) pe12 ON pe12.PersonId=pe.PersonId
INNER JOIN Person ON Id=pe.PersonId
WHERE pe.DateStarted=TheMostEventDate
For the last event date and previous event:
SELECT ID,NAME,TheMostEventDate,ReasonForLeaving
FROM PersonEvents pe
INNER JOIN(
SELECT pe1.PersonId,TheMostEventDate,NextToMostEventDate=MAX(pe1.DateStarted)
FROM PersonEvents pe1
INNER JOIN(
SELECT PersonId,TheMostEventDate=MAX(DateStarted)
FROM PersonEvents
GROUP BY PersonId
) pe2
ON pe2.PersonId=pe1.PersonId
WHERE DateStarted<TheMostEventDate
GROUP BY pe1.PersonId,TheMostEventDate
) pe12 ON pe12.PersonId=pe.PersonId
INNER JOIN Person ON Id=pe.PersonId
WHERE pe.DateStarted=NextToMostEventDate