In SQL how do I write a query to return 1 record from a 1 to many relationship? - sql

Question

Let's say I have a Person table and a Purchases table with a 1 to many relationship. I want to run a single query that returns this person and just their latest purchase. This seems easy but I just can't seem to get it.

Answer 1

select p.*, pp.*
from Person p
left outer join (
select PersonID, max(PurchaseDate) as MaxPurchaseDate
from Purchase
group by PersonID
) ppm
left outer join Purchase pp on ppm.PersonID = pp.PersonID
and ppm.MaxPurchaseDate = pp.PurchaseDate
where p.PersonID = 42
This query will also show the latest purchase for all users if you remove the WHERE clause.

Answer 2

Assuming you have something like a PurchaseDate column and want a particular person (SQL Server):
SELECT TOP 1 P.Name, P.PersonID, C.PurchaseDescription FROM Persons AS P
INNER JOIN Purchases AS C ON C.PersonID = P.PersonID
WHERE P.PersonID = #PersonID
ORDER BY C.PurchaseDate DESC

Answer 3

Many Databases preform the "Limit or Top" command in different ways. Here is a reference http://troels.arvin.dk/db/rdbms/#select-limit and below are a few samples
If using SQL Server
SELECT TOP 1
*
FROM Person p
INNER JOIN Purchases pc on pc.PersonID = P.PersonID
Order BY pc.PurchaseDate DESC
Should work on MySQL
SELECT
*
FROM Person p
INNER JOIN Purchases pc on pc.PersonID = P.PersonID
Order BY pc.PurchaseDate DESC
LIMIT 1

Answer 4

Strictly off the top of my head!...If it's only one record then...
SELECT TOP 1 *
FROM Person p
INNER JOIN Purchases pu
ON p.ID = p.PersonId
ORDER BY pu.OrderDate
WHERE p.ID = *thePersonYouWant*
otherwise...
SELECT TOP 1 *
FROM Person p
INNER JOIN
(
SELECT TOP 1 pu.ID
FROM Purchases pu
ON pu.PersonID = p.Id
ORDER BY pu.OrderDate
) sq
I think! I haven't got access to a SQL box right now to test it on.

Answer 5

Without knowing your structure at all, or your dbms, you would order the results descending by the purchase date/time, and return only the first joined record.

Answer 6

Try TOP 1 With an order by desc on date. Ex:
CREATE TABLE #One
(
id int
)
CREATE TABLE #Many
(
id int,
[date] date,
value int
)
INSERT INTO #One (id)
SELECT 1 UNION ALL
SELECT 2 UNION ALL
SELECT 3
INSERT INTO #Many (id, [date], value)
SELECT 1, GETDATE(), 1 UNION ALL
SELECT 1, DATEADD(DD, 1 ,GETDATE()), 3 UNION ALL
SELECT 1, DATEADD(DD, -1 ,GETDATE()), 0
SELECT TOP 1 *
FROM #One O
JOIN #Many M ON O.id = M.id
ORDER BY [date] DESC

Answer 7

If you want to select the latest purchase for each person, that would be:
SELECT PE.ID, PE.Name, MAx(PU.pucrhaseDate) FROM Persons AS PE JOIN PURCHASE as PU ON PE.ID = PU.Person_ID
If you want to have all persons also those who have no purchases, you need to use LEFT JOIN.

Answer 8

I think you need one more table called Items for example.
The PERSONS table would uniquely define each person and all their attributes, while the ITEMS table would uniquely define each items and their attributes.
Assume the following:
Persons |Purchases |Items
PerID PerName |PurID PurDt PerID ItemID |ItemID ItemDesc ICost
101 Joe Smith |201 101107 101 301 |301 Laptop 500
|202 101107 101 302 |302 Desktop 699
102 Jane Doe |203 101108 102 303 |303 iPod 199
103 Jason Tut |204 101109 101 304 |304 iPad 499
|205 101109 101 305 |305 Printer 99
One Person Parent may tie to none, one or many Purchase Child.
One Item Parent may tie to none, one or many Purchase Child.
One or more Purchases Children will tie to one Person Parent, and one Item Parent.
select per.PerName as Name
, pur.PurDt as Date
, itm.ItemDesc as Item
, itm.ICost as Cost
from Persons per
, Purchases pur
, Items itm
where pur.PerID = per.PerID -- For that Person
and pur.ItemID = itm.ItemID -- and that Item
and pur.PurDt = -- and the purchase date is
( Select max(lst.PurDt) -- the last date
from Purchases lst -- purchases
where lst.PerID = per.PerID ) -- for that person
This should return:
Name Date Item Cost
Joe Smith 101109 Ipad 499
Joe Smith 101109 Printer 99
Jane Doe 101108 iPod 199