Is there any alternative way to write this t-sql query? - sql

Question

I have these 3 tables:
For each car I need to visualize the data about the last (most recent) reservation:
the car model (Model);
the user who reserved the car (Username);
when it was reserved (ReservedOn);
when it was returned (ReservedUntil).
If there is no reservation for a given car, I have to show only the car model. Other fields must be empty.
I wrote the following query:
SELECT
Reservations.CarId,
res.MostRecent,
Reservations.UserId,
Reservations.ReservedOn,
Reservations.ReservedUntil
FROM
Reservations
JOIN (
Select
Reservations.CarId,
MAX(Reservations.ReservedOn) AS 'MostRecent'
FROM
Reservations
GROUP BY
Reservations.CarId
) AS res ON res.carId = Reservations.CarId
AND res.MostRecent = Reservations.ReservedOn
This first one works but I got stuck to obtain the result that I need. How could I write complete the query?

Answer 1

It looks like a classic top-n-per-group problem.
One way to do it is to use OUTER APPLY. It is a correlated subquery (lateral join), which returns the latest Reservation for each row in the Cars table. If such reservation doesn't exist for a certain car, there will be nulls.
If you create an index for Reservations table on (CarID, ReservedOn DESC), this query should be more efficient than self-join.
SELECT
Cars.CarID
,Cars.Model
,A.ReservedOn
,A.ReservedUntil
,A.UserName
FROM
Cars
OUTER APPLY
(
SELECT TOP(1)
Reservations.ReservedOn
,Reservations.ReservedUntil
,Users.UserName
FROM
Reservations
INNER JOIN Users ON Users.UserId = Reservations.UserId
WHERE
Reservations.CarID = Cars.CarID
ORDER BY
Reservations.ReservedOn DESC
) AS A
;
For other approaches to this common problem see Get top 1 row of each group
and Retrieving n rows per group

Answer 2

With not exists:
select r.* from reservations r
where not exists (
select 1 from reservations
where carid = r.carid and reservedon > r.reservedon
)
You can create a CTE with the above code and join it to the other tables:
with cte as (
select r.* from reservations r
where not exists (
select 1 from reservations
where carid = r.carid and reservedon > r.reservedon
)
)
select c.carid, c.model, u.username, cte.reservedon, cte.reserveduntil
from cars c
left join cte on c.carid = cte.carid
left join users u on u.userid = cte.userid
If you don't want to use a CTE:
select c.carid, c.model, u.username, t.reservedon, t.reserveduntil
from cars c
left join (
select r.* from reservations r
where not exists (
select 1 from reservations
where carid = r.carid and reservedon > r.reservedon
)
) t on c.carid = t.carid
left join users u on u.userid = t.userid