Selecting multiple fields from row with max value of column, per group - sql

Question

I'm quite certain I've painted myself into a corner and I can't figure my way out.
The Users table and OrderHistories tables both have 1+ million records:
SELECT
u.Id ,
u.Email AS EmailAddress ,
c.Address_Address1 AS "Address 1" ,
(
SELECT
COUNT(*)
FROM
dbo.OrderHistories oh
WHERE
oh.UserId = u.UserName
) AS NumberOfOrders ,
Carts.SubtotalAmount AS CartTotal ,
(
SELECT
MAX(oh.CreateDate)
FROM
dbo.OrderHistories AS oh
WHERE
oh.UserId = u.Id
) AS LastOrderDate ,
(
SELECT
LastOrders.SubtotalAmount AS LastOrderSubtotal
FROM
(
SELECT
UserId ,
CreateDate ,
SubtotalAmount ,
MAX(CreateDate) OVER ( PARTITION BY UserId ) MyLastOrderDate
FROM
Users u
INNER JOIN dbo.OrderHistories oh
ON u.Id = oh.UserId
) AS LastOrders
WHERE
LastOrders.MyLastOrderDate = LastOrders.CreateDate
AND LastOrders.UserId = u.Id
) AS LastOrderSubtotal
FROM
Users u
INNER JOIN Customers AS c
ON u.Id = c.Id
LEFT JOIN dbo.Carts
ON c.Id = Carts.CustomerId
This particular subquery is my current problem (EXTREMELY inefficient), but I'm not experienced enough to understand exactly why, or how I should be doing it instead (I can't get there from here!):
(
SELECT
LastOrders.SubtotalAmount AS LastOrderSubtotal
FROM
(
SELECT
UserId ,
CreateDate ,
SubtotalAmount ,
MAX(CreateDate) OVER ( PARTITION BY UserId ) MyLastOrderDate
FROM
Users u
INNER JOIN dbo.OrderHistories oh
ON u.Id = oh.UserId
) AS LastOrders
WHERE
LastOrders.MyLastOrderDate = LastOrders.CreateDate
AND LastOrders.UserId = u.Id
) AS LastOrderSubtotal
Anyone mind telling me how terrible I am and then segue right into a suggested improvement?

Answer 1

Just from looking at your query, you may be able to simplify it using cross apply() like so:
select
u.Id
, EmailAddress = u.Email
, [Address 1] = c.Address_Address1
, CartTotal = Carts.SubtotalAmount
, NumberOfOrders = oh.NumberOfOrders
, LastOrderDate = oh.CreateDate
, LastOrderSubtotal = oh.SubtotalAmount
from Users u
inner join Customers AS c
on u.Id = c.Id
left join dbo.Carts
on c.Id = Carts.CustomerId
cross apply (
select top 1
i.CreateDate
, i.SubtotalAmount
, NumberOfOrders = count(*) over (partition by i.UserId)
from dbo.OrderHistories i
where i.UserId = u.Id
order by i.CreateDate desc
) as oh
If you want rows that may not have an OrderHistory, switch to outer apply().
Reference:
apply() - msdn
The power of T-SQL's APPLY operator - Rob Farley
APPLY: It Slices! It Dices! It Does It All! - Brad Shulz