Getting oldest Date SQL Complexity - sql

Question

I have a problem which I cannot resolve no matter what without using code, instead of SQL SCRIPT.
I have 2 tables
Person
ID Name Type
1 A A1
2 B A2
3 C A3
4 D A4
5 E A6
PersonHomes
HOMEID Location PurchaseDate PersonID
1 CA 20160101 1
2 CT 20160202 1
3 DT 20160101 2
4 BT 20170102 3
5 CT 20160303 1
6 CA 20160101 2
PersonID is foreign key of Person Table
There are no other rowz in the tables
So, we have to show detail of EACH person WITH home
The rule to write output is
IF Person has SINGLE entry in PersonHomes then use it
IF Person has MORE than ONE entry in PersonHomes then we have to look at purchase date, IF they are different then USE the PersonHomes ROW with OLDEST date in it. AND DELETE OTHER ROWS OF HIM
IF Person has MORE than ONE entry in PersonHomes then we have to look at purchase date, and IF DATES are SAME then USE the ROW with LOWER ID AND DELETE THE OTHER ROWS of HIM
This is very easy to do in code but using SQL it is complex
What I tried was to
WITH PERSON (
SELECT * FROM Person)
SELECT * FROM PERSON
INNER JOIN PersonHomes ON Person.ID = PersonHomes.PersonID
WHERE PersonHomes.PersonID = CASE WHEN (COUNT (*) FROM PersonHomes...)
Then I think I can write SQL function ?
I am stuck, Please help!
SAMPLE OUTPUT for PERSON A
ID NAME Type HOMEID Location PurchaseDate
1 A A1 5 CT 20160303
For PERSON B
ID NAME Type HOMEID Location PurchaseDate
1 A A2 3 DT 20160101
Aiden

Answer 1

It is not so easy to get desired output with SQL. we should write more than one sql queries.
First I created a temp table which consists of home details:
select PersonID, count(*) as HomeCount, count(distinct PurchaseDate) as
PurchaseDateCount, min(PurchaseDate) oldestPurchaseDate, min(HOMEID) as
LowerHomeID into #PersonHomesAbstractTable from PersonHomes group by PersonID
Then for the output of your first rule:
select p.ID, p.NAME, p.Type, ph.HOMEID, ph.Location, ph.PurchaseDate from Person p
inner join #PersonHomesAbstractTable a on p.ID = a.PersonID
inner join PersonHomes ph on p.ID = ph.PersonID
where a.HomeCount = 1
For the output of your second rule:
select p.ID, p.NAME, p.Type, ph.HOMEID, ph.Location, ph.PurchaseDate
from Person p inner join #PersonHomesAbstractTable a on p.ID = a.PersonID
inner join PersonHomes ph on p.ID = ph.PersonID and
ph.PurchaseDate = a.oldestPurchaseDate
where a.HomeCount > 1 and a.PurchaseDateCount <> 1
And finally for the output of your third rule:
select p.ID, p.NAME, p.Type, ph.HOMEID, ph.Location, ph.PurchaseDate
from Person p inner join #PersonHomesAbstractTable a on p.ID = a.PersonID
inner join PersonHomes ph on p.ID = ph.PersonID and
ph.HOMEID = a.LowerHomeID
where a.HomeCount > 1 and a.PurchaseDateCount = 1
Of course there are some other ways, but now this way is come to my mind.
If you want to delete undesired rows, you can use scripts below:
delete from PersonHomes where HOMEID in
(
select ph.HOMEID from #PersonHomesAbstractTable a
inner join PersonHomes ph on a.PersonID = ph.PersonID and
ph.PurchaseDate <> a.oldestPurchaseDate
where a.HomeCount > 1 and a.PurchaseDateCount <> 1
union
select p.HOMEID from #PersonHomesAbstractTable a
inner join PersonHomes ph on a.PersonID = ph.PersonID and
ph.HOMEID <> a.LowerHomeID
where a.HomeCount > 1 and a.PurchaseDateCount = 1
)

Answer 2

You seem to have a prioritization query. I would solve this using row_number():
select ph.*
from (select ph.*,
row_number() over (partition by personid
order by purchasedate asc, homeid asc
) as seqnum
from personhomes ph
) ph
where seqnum = 1;
This doesn't actually change the data in the table. Although you say delete, it seems like you just want a result set with one home per person.

Answer 3

This is shortest approach got by Link
;WITH cte AS
(
SELECT *, RowN = ROW_NUMBER() OVER (PARTITION BY ID ORDER BY AddressMoveDate DESC) FROM Address
)
DELETE FROM cte WHERE RowN > 1