Complex SQL query, not sure where to start - sql-server

Question

I have a tough one here I think. I have the following tables:
[Assets]
AssetId | Name
1 | Acura NSX
2 | Dodge Ram
[Assignments]
AssignmentId | AssetId | StartMileage | EndMileage | StartDate | EndDate
1 | 1 | 8000 | 10000 | 4/1/2015 | 5/1/2015
2 | 1 | 10000 | 16000 | 9/15/2015 | 1/5/2016
3 | 2 | 51000 | NULL | 1/1/2016 | NULL
[Reminders]
ReminderId | AssetId | Name | Distance | Time | Active
1 | 1 | Oil Change | 3000 (miles)| 3 (months)| 1
2 | 1 | Tire Rotation | 5000 | 6 | 0
3 | 2 | Oil Change | 3000 | 3 | 1
4 | 2 | Air Filter | 50000 | 48 | 1
[Maintenance]
MaintenanceId | AssetId | ReminderId | Mileage | Date | Vendor
1 | 1 | 1 | 10000 | 5/1/2015 | Jiffy Lube
2 | 2 | 3 | 51000 | 6/1/2015 | Dealership
I need a query that will join these 4 tables and return something like the following.
Name | Name | Current Mileage | Last Mileage | Last Date
Acura NSX | Oil Change | 16000 | 10000 | 5/1/2015
Dodge RAM | Oil Change | 51000 | 51000 | 6/1/2015
Dodge RAM | Air Filter | 51000 | -- | --
I need to take the distance threshold from the Reminders table and add it to the mileage from the Maintenance table then compare it to the start and end mileage from the Assignments table. If the threshold is greater than the start or end mileage then select the asset name, the name of the reminder, the current mileage (start or end mileage from Assignments, whichever is greater), and mileage and date from the last maintenance for that reminder. I need to do the same for time threshold. Add it to the date from the Maintenance table then compare it to today's date. If it's greater then display the asset.
Can one of you SQL gurus help me with this please?
UPDATE:
SELECT
v.Name,
r.Name AS Reminder,
a.CurrentMileage,
i.MaintenanceMileage,
i.MaintenanceDate
FROM
Assets v
LEFT JOIN
(SELECT AssetId,
COALESCE(EndMileage, StartMileage) AS CurrentMileage,
ROW_NUMBER() OVER (PARTITION BY AssetId
ORDER BY AssignmentId DESC) AS window_id
FROM Assignments) a
ON v.AssetId = a.AssetId
AND a.window_id = 1
JOIN
Reminders r
ON v.AssetId = r.AssetId
AND r.ActiveFlag = 1
LEFT JOIN
(SELECT AssetId,
ReminderId,
MAX(Mileage) AS MaintenanceMileage,
MAX([Date]) AS MaintenanceDate
FROM Maintenances
GROUP BY AssetId, ReminderId) i
ON r.ReminderId = i.ReminderId
AND (a.CurrentMileage > (NULLIF(i.MaintenanceMileage, 0) + r.DistanceThreshold))
OR (GETDATE() > DATEADD(m, r.[TimeThreshold], i.MaintenanceDate))

Answer 1

Here is a starting point:
SELECT v.Name AS [Asset Name], r.Name AS Reminder, a.CurrentMileage,
m.Mileage + r.Distance AS [Last Mileage], m.[Date] AS [Last Date]
FROM Assets v
JOIN ( -- get the latest relevant row as window_id = 1
SELECT AssetId, COALESCE(EndMileage, StartMileage) AS CurrentMileage,
COALESCE(EndDate, StartDate) AS AssignDate,
ROW_NUMBER() OVER (partition by AssetId
order by COALESCE(EndDate, StartDate) DESC) AS window_id
FROM Assignments
) a
ON v.AssetId = a.AssetId
AND a.window_id = 1
JOIN Reminders r
ON v.AssetId = r.AssetId
AND r.Active = 1
LEFT JOIN Maintenance m
ON r.AssetId = m.AssetId
AND r.ReminderId = m.ReminderId
-- corrected
AND ((a.CurrentMileage > (NULLIF(m.Mileage, 0) + r.Distance))
-- slightly oversimplified
OR (GETDATE() > DATEADD(m, r.[Time], COALESCE(m.[Date], a.AssignDate))))
The date calculations are slightly oversimplified because they use the latest assignment dates. What you would really want is a column Assets.InServiceDate that would anchor the time before the first maintenance would be due. But this will get you started.